Semiconductor memory device

ABSTRACT

Disclosed is a semiconductor device, such as a semiconductor memory device, having structure wherein invasion of minority carriers from the semiconductor substrate into components of the device, formed on the substrate, can be avoided. The semiconductor memory device can be an SRAM or DRAM, for example, and includes a memory array and peripheral circuit on a substrate. In one aspect of the present invention, a buried layer of the same conductivity type as that of the substrate, but with a higher impurity concentration than that of the substrate, is provided beneath at least one of the peripheral circuit and memory array. A further region can extend from the buried layer, for example, to the surface of the semiconductor substrate, the buried layer and further region in combination acting as a shield to prevent minority carriers from penetrating to the device elements. As a second aspect of the present invention, first carrier absorbing areas (to absorb minority carriers) are located between the memory array and the switching circuit of the peripheral circuit, and second carrier absorbing areas are provided to surround input protective elements of the device. As a third embodiment of the present invention, a plurality of isolation regions of the same conductivity type are provided, with unequal voltages applied to these isolation regions, or unequal voltages applied to the substrate, on the one hand, and to these isolation regions, on the other.

[0001] This application is a continuation-in-part application ofapplication Ser. No. 889,405, filed Aug. 26, 1986; acontinuation-in-part application of application Ser. No. 087,256, filedJul. 13, 1987; and a continuation-in-part application of applicationSer. No. 029,681, filed Mar. 24, 1987.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a semiconductor integratedcircuit device which has a bipolar transistor and a MISFET(Metal-Insulator-Semiconductor Field Effect Transistor). Moreparticularly, it relates to techniques which are effective when appliedto a memory device, such as a DRAM (Dynamic Random Access Memory) deviceor a SRAM (Static Random Access Memory) device having bipolartransistors, particularly both bipolar transistors and MISFETs.

[0003] Even more particularly, it relates to a random access memory withhigh speed and low consumption of power, where a switching circuit as acomplex circuit of a bipolar transistor and metal-oxide semiconductorfield effect transistor (hereinafter referred to as “MOSFET”) is adoptedas a peripheral circuit (address circuit, timing circuit or the like) ofthe memory. Moreover, this invention relates to techniques for isolationbetween elements of the device.

[0004] Generally, the present invention is directed to techniques insemiconductor memories, such as Bi-CMOS (bipolartransistor-complementary metal-oxide-semiconductor structure) memories,to avoid destruction of information due to minority carriers.

[0005] Semiconductor memories are manufactured as products of largecapacity, such as 64K bits, 256K bits, in recent years. Muchdevelopmental work has been done on various semiconductor memories, suchas dynamic random access memories (DRAMs) and static random accessmemories (SRAMs).

[0006] The so-called one-MOSFET type memory cell, which is composed ofone capacitor for storing information charges and one MOS(Metal-Oxide-Semiconductor) FET for switching, has a small occupationarea and is suited to raise the density of integration. Therefore, it isextensively adopted as the memory cell of a DRAM.

[0007] In the DRAM, circuits other than a memory cell array, namely,peripheral circuits such as various timing generators, an address buffercircuit, an address decoder circuit, data input/output circuits, a senseamplifier and a main amplifier are constructed of CMOS (ComplementaryMOS) circuits in each of which an N-channel MOSFET and a P-channelMOSFET are combined. Thus, the DRAM is permitted to exhibit a lowerpower consumption as well as a higher operating speed and to have ahigher density of integration. The DRAM which employs the CMOS circuitsfor the peripheral circuits is described in, for example, “NikkeiElectronics,” Jul. 18, 1983, pp. 188-190.

[0008] In order to meet the needs of the age for memories of largecapacity, investigations have been made from a viewpoint ofmanufacturing memories with large capacity, high speed and lowconsumption of power. Illustratively, in order to attain a still higheroperating speed and higher integration density, devices constituting aDRAM need to be made smaller, but the magnitudes of signals to betreated decrease with the smaller devices. In order to treat the smallsignal magnitude at high speed, a high drivability is required of theconstituent device of the circuitry. However, insofar as a CMOS circuitis used as the device, the sizes of MOSFETs cannot be made very largefrom the viewpoint of the density of integration, and the drivability(conductance g_(m)) of each MOSFET is low, so that the operating speedof the memory lowers along with the density of integration. As a result,development of a memory in a mixed state of bipolar transistor andcomplementary metal-oxide semiconductor field effect transistors(CMOSFET) (the memory being hereinafter referred to as “Bi-CMOSmemory”), as shown in Japanese patent application No. 22811/1984(corresponding to U.S. patent application Ser. No. 701,226), has beenconsidered. Specifically, in order to simultaneously achieve the higherdensity of integration and the higher operating speed, we have madestudies of using a bipolar transistor in the peripheral circuit of theDRAM.

[0009]FIG. 29 illustrates a fundamental sectional structure of a BI-CMOSsystem. Of course, the system shown is merely exemplary. Such Bi-CMOSsystem is stated in detail in “Nikkei Electronics”, Aug. 12, 1985, pp.187-208. Shown in the figure are one n-channel MOS (nMOS) transistor, aswell as one p-channel MOS (pMOS) transistor, and an n-p-n bipolar(npnBIP) transistor.

[0010] Here, letters S, G and D affixed to the nMOS or pMOS indicate thenodes of the source, gate and drain thereof, respectively, while lettersC, E and B affixed to the npnBIP transistor, indicate the nodes of thecollector, emitter and base thereof, respectively. Besides, in thefigure, diffusion layers have only the impurity types thereof writtendown for the-sake of brevity. Accordingly, as regards portions to whichthe same symbols are assigned, it is merely indicated that theconductivity types are the same, and the impurity materials and impurityconcentrations are selected at will properly according to the purposesof the portions.

[0011] The Bi-CMOS memory will now be described briefly.

[0012] In an address circuit, a timing circuit or the like as aperipheral circuit within a semiconductor memory, an output transistorfor charging and discharging parasitic capacitance in signal lines oflong distance, and an output transistor with large fan-out areconstituted by bipolar transistors, and a logical circuit for performinglogical processing such as inversion, non-inversion, NAND, NOR isconstituted by a CMOS circuit. The logical circuit constituted by theCMOS circuit is of low power consumption, and an output signal of thelogical circuit is transmitted through the bipolar output transistorwith low output impedance to the signal lines of long distance. Sincethe output signal is transmitted to the signal lines using the bipolaroutput transistor with low output impedance, dependence of the signalpropagation delay time on the parasitic capacitance of the signal linescan be reduced, whereby a semiconductor memory with low consumptionpower and high speed is obtained.

[0013] However, as discussed further below, problems arise in usingbipolar transistors in the peripheral circuits of, for example, theDRAM.

[0014] Heretofore, in an integrated circuit employing insulated-gatefield effect transistors (hereinafter, abbreviated to “MOS transistors”)or bipolar transistors (hereinafter, abbreviated to “BIP transistors”),isolation among the elements of the integrated circuit has beenperformed by applying reverse bias voltages to p-n junctions. Thedetails are stated in, for example, “Integrated Circuit Technology (1)”(Corona Publishing Co., Ltd.) by Yanai and Nagata, pp. 21-31. In aBi-CMOS system, a similar device isolation method similar to the aboveis adopted.

[0015] In such a Bi-CMOS system, in the-prior art, the isolation among alarge number of devices within a chip is executed by applying the lowestpotential in the circuitry to a p-type substrate (p-Sub) and the highestpotential in the circuity to the:-n-type-isolation-layer (nWELL) forforming the pMOS transistor, whereby the junctions of various parts areprevented from falling into the condition of forward bias. That is, withthe prior art, in a case where the circuitry operates between a supplyvoltage (for example, 5 V) and the earth (0 V), the devices are isolatedby applying 0 V to the substrate p-Sub and 5 V to the n-type isolationlayer. Since, in such a system, the applied voltage to the substratep-Sub or to the n-type isolation layer is selected at the lowest voltagerequired for the device isolation, reverse bias voltages to be appliedto the p-n junctions can be rendered small, and therefore it is possibleto cope with the problems of lowering in-the breakdown voltages ofdevices, etc. attendant upon the future microminiaturization of thedevices.

PROBLEMS FOUND AND ADDRESSED IN THE PRESENT INVENTION

[0016] In studying Bi-CMOS memories, the inventors have found variousmodes for destruction of information in the memory. Thus, the inventorshave studied the information destruction modes of the information storedin the memory cell of the Bi-CMOS memory, and have found a novelinformation destruction mode as hereinafter described and have completedone aspect of the present invention as a means for preventing suchinformation destruction.

[0017] In addition, the inventors have found that, in a memory deviceincluding a bipolar transistor, minority carriers created by theexistence of the bipolar transistor incur the so-called soft error inwhich they invert information stored in a memory cell or informationread out from a memory cell to a data line.

[0018] The mechanism of the soft error attributed to the bipolartransistor is interpreted as below by way of example.

[0019] The switching MOSFET of the memory cell (exemplified by, but notlimited to, a DRAM) is constructed of an N-channel MOSFET which isformed within a p⁻-type substrate. The capacitor of the memory cell hasan n⁺-type semiconductor region being one electrode thereof within thep⁻-type substrate. Meanwhile, as a desirable device for attaining a highdrivability at high speed, a vertical npn-type bipolar transistor isconstructed of an n⁺-type emitter region, a p-type base region, andn⁻-type and n⁺-type collector regions within the p⁻-type substrate. Inorder to lead the electrode of the collector of this bipolar transistorout of the surface of the substrate, the n⁺-type buried collector regionis formed larger (longer) than the emitter region. For this reason, apotential fluctuation is liable to occur within the buried collectorregion due to a resistance possessed by itself. The potentialfluctuation of the buried collector region causes a pnp-type parasiticbipolar transistor to operate and injects holes into the substrate, toinduce a fluctuation in the potential of the substrate. The parasiticbipolar transistor appears with the base region as its emitter region,the buried collector region as-its base region and the substrate as itscollector region. On account of the fluctuation of the substratepotential, the substrate is injected with electrons (minority carriers)from an n⁺-type semiconductor region of high impurity concentration nearthe parasitic bipolar transistor (for example, the source region ordrain region of an N-channel MOSFET). The minority carriers enter then⁺-type region of the switching MOSFET and capacitor of the memory cellor the n⁺-type region of the MOSFET of a sense amplifier or the like,and invert (destroy) information to give rise to the so-called softerror.

[0020] Access-time of the DRAM can be shortened by incorporating bipolartransistors into the peripheral circuits. At the same time, however, thesoft errors develop conspicuously due to the minority carriers which arecreated within the substrate by the bipolar transistors or alphaparticles. More specifically, the number of times that informationpasses between a data line and the capacitor increases, to consequentlyheighten the probability of the minority carriers being trappedparticularly in the source region or drain region of the switchingMOSFET.

[0021] Accordingly, when the bipolar transistors are caused to coexistin the DRAM with the intention of attaining a higher operating speed anda higher density of integration, there is involved the problem that theelectrical reliability of the DRAM lowers due to the soft errors.

[0022] A further problem addressed by the present invention involvesinjection of minority carriers arising from application of reversebiasing to provide isolation between semiconductor elements of theintegrated circuit device. Since the input or output terminal of an LSI(large-scale integrated circuit) is directly connected with an externalcircuit, noise above the supply voltage or below 0 V (in general, surgenoise such as overshoot or undershoot) might arise. Since the input oroutput node is connected to the diffusion layer within the chip in anyform, the corresponding junction will then be forward-biased in theprior art. By way of example, when the minus surge noise is applied tothe n-type diffusion layer indicated at the source S or drain D of thenMOS transistor in FIG. 29, the junction between the n-type diffusionlayer and the substrate p-Sub is forward-biased, and a forward currentflows from the substrate p-Sub toward the n-type diffusion layer. As aresult, minority carriers (electrons in a p-type substrate) are injectedinto the substrate p-Sub. Since the minority carriers have a mean freepath which is usually as long as several hundred μm, they reach anothercircuit part and incur, for example, the problem that a stored signal ina memory cell is destroyed in an SRAM or DRAM. The phenomenon of theminority carrier injection might be caused not only at the input oroutput node part, but also by a circuit operation inside the chip insuch a manner that the potential of the diffusion layer or of thesubstrate p-Sub fluctuates locally due to capacitive coupling or due tosaturated operation of the bipolar transistor. It is thereforeimpossible to realize high performance of the Bi-CMOS system.

SUMMARY OF THE INVENTION

[0023] An object of the present invention is to provide a semiconductorintegrated circuit device which is immune against soft errors, which ishigh in the density of integration and low in power consumption andwhich is suited to a high speed operation, and a method of manufacturingthe same.

[0024] Another object of the present invention is to provide a techniquewhich can raise the operating speed and enhance the electricalreliability of a DRAM, or other memory device, having bipolartransistors.

[0025] A further object of the invention is to provide a semiconductormemory technique, wherein destruction of storage information caused byuse of a Bi-CMOS logic circuit (switching circuit) in peripheralcircuitry can be prevented, and also destruction of storage informationdue to other factors can be prevented.

[0026] A still further object of the present invention is to solve theproblem arising due to reverse-biasing to provide isolation betweenelements, and to provide a semiconductor device which operates stably.Another object of the present invention is to provide a voltageapplication method by which a voltage to be applied to a substrate or anisolation region is freely set according to an intended use, as well asa device structure which makes the method possible.

[0027] The aforementioned and other objects and novel features of thepresent invention will become apparent from the description of thespecification and the accompanying drawings.

[0028] Typical ones of the aspects of the present invention disclosed inthe present-application will be summarized below.

[0029] Under the memory cell of a memory device (illustratively a DRAM)having bipolar transistors and/or under the semiconductor region of theperipheral circuit thereof, a semiconductor region which has the sameconductivity type as that of a substrate and an impurity concentrationhigher than that of the substrate is disposed. That is, a semiconductorregion for forming the circuit element of the memory cell, or thesemiconductor region of the circuit element of the peripheral circuit,is underlaid with a semiconductor region the conductivity type of whichis opposite to that of the former semiconductor region. Moreover, otherregions (e.g., a further semiconductor region) can be used incombination with the underlying semiconductor region to act as a shieldto prevent minority carriers from entering the memory cell and/or fromentering the semiconductor region electrically connected to the bitline. For example, a further semiconductor region can extend from theunderlying semiconductor region to the surface of the semiconductorsubstrate, for example, the underlying semiconductor region and thefurther semiconductor region together acting as a shield to preventminority carriers from entering the memory cell and/or bit line.

[0030] Thus, where the substrate is of p-type conductivity and theunderlying layer is a p⁺-type buried layer, a p-type region can beprovided extending from the p⁺-type buried layer to the substratesurface to act as such shield. For example, such p-type region can formpart of the isolation region between the memory cell and n-type MISFETof the peripheral circuit, the p-type region extending from thep⁺-buried layer to a field oxide film formed on the semiconductorsubstrate. Such p-type region, in combination with the underlyingp⁺-buried layer, can act as a shield both for cell mode soft error(e.g., soft error due to introduction of minority carriers into thecapacitor of the one-MISFET type memory cell) and bit line mode softerror (e.g., soft error due to minority carriers entering the drain ofthe switching MISFET of the one-MISFET type memory cell).

[0031] As a further illustrative example, the underlying semiconductorregion (for example, p⁺-type buried layer) can be provided to contact aregion of opposite conductivity type extending to the substrate surface(for example, an n-type well layer) so as to provide the shieldingfunction.

[0032] Such combination of regions (for example, the p⁺-type buriedlayer and p-type region; or the p⁺-type buried layer and n-type well)can act to prevent soft error caused by minority carriers generated bythe bipolar transistor (that is, parasitic bipolar transistor action).

[0033] In addition, a p⁺-type buried layer is provided under the drainregion of the switching MISFET of the memory cell, and under the MISFETof a sense amplifier of the peripheral circuitry, to avoid bit line modesoft error generated by α-particles.

[0034] According to the above expedient, the bipolar transistor iscaused to coexist in the peripheral circuit of the DRAM, thereby toattain a higher operating speed, and the underlying semiconductor regionand further region form a potential barrier to minority carriers createdby the bipolar transistor, thereby to prevent soft errors ascribable tothe minority carriers.

[0035] A further aspect of the present invention will be set forth. Afirst carrier absorbing area is formed between a peripheral circuit unitwith a Bi-CMOS complex circuit as a switching circuit and a memory cellarray unit so as to prevent minority carriers (electrons) from moving.The first carrier absorbing area has a function to reduce the effectivecurrent amplification factor of the parasitic bipolar transistor formedbetween the peripheral circuit unit and the memory cell array unit. As aresult, partial destruction of the storage information caused by use ofthe Bi-CMOS type logical circuit in the peripheral circuit can beprevented.

[0036] A still further aspect of the present invention will be setforth. In the present invention, a voltage which is still negative (ingeneral, when a p-type silicon substrate is used) or positive (ingeneral, when an n-type silicon substrate is used) with respect to theoperating voltage of circuitry is applied to a portion liable to theinjection of minority carriers, for example, a substrate. Further, inthe present invention, in order to solve problems ascribable to themethod of applying the voltage as stated above, for example, the problemthat voltages to be applied to individual devices increase to degradethe reliability of a device of low breakdown voltage such as amicrominiature device, the isolation region of MOS transistors ofidentical conductivity type or bipolar transistors of identicalconductivity type is divided into several isolated regions, to whichsuitable isolation voltages are applied according to the respectiveuses.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIGS. 1-3 are sectional views each showing the structure of aDRAM having bipolar transistors in accordance with a first aspect of thepresent invention;

[0038]FIG. 4 is-a graph showing the soft error rates of the DRAMs inFIGS. 1-3;

[0039]FIG. 5 is a sectional view showing another structure of aP-channel MOSFET which is included in each of the DRAMs in FIGS. 1-3;

[0040] FIGS. 6-10 are sectional views each showing other structures of amemory cell and an N-channel MOSFET which are included in each of theDRAMs in FIGS. 1-3;

[0041]FIGS. 11 and 12 are sectional views each showing the structure ofa DRAM having bipolar transistors which is another embodiment of thisfirst aspect of the present invention;

[0042] FIGS. 13A-13D are sectional views showing the outline of amanufacturing process for the DRAM in FIG. 11;

[0043]FIG. 14 is a circuit diagram illustrating a memory cellinformation destruction mechanism clarified by the inventors;

[0044]FIG. 15 is a device plane layout diagram corresponding to a partof the circuit diagram of FIG. 14;

[0045]FIG. 16 is a device sectional view corresponding to the circuitdiagram of FIG. 14 and the layout diagram of FIG. 15;

[0046]FIG. 17 is a circuit diagram illustrating another memory cellinformation destruction mechanism clarified by the inventors;

[0047]FIG. 18 is a device sectional view of input protective elementM105 and MOSFET M111 of memory cell MC111 shown in FIG. 17;

[0048]FIG. 19 is a circuit diagram corresponding to FIG. 14 in appliedstate of the invention;

[0049]FIG. 20 is a device plane layout diagram in an applied state of asecond aspect of the invention where first minority carrier absorbingarea 113 connected to definite potential is formed between peripheralcircuit 1110 and memory array 1120;

[0050]FIG. 21 is a device sectional view in applied state of a firstembodiment of this second aspect of the invention;

[0051]FIG. 22 is a whole device plane layout diagram of a memory inapplied state of the first embodiment of this second aspect of theinvention;

[0052]FIG. 23 is a sectional view of a modification of the firstembodiment of this second aspect of the invention, where first minoritycarrier absorbing area 113 (161) is formed simultaneously tosource/drain layer of NMOSFET;

[0053]FIG. 24 is a device sectional view of a second embodiment of thissecond aspect of the invention, illustrating a second minority carrierabsorbing area 114 formed to surround input protective element M105 andfirst minority carrier absorbing area 113 formed between peripheralcircuit 1110 and memory array 1120;

[0054]FIG. 25 is a device sectional view of a modification of FIG. 24,where n type buried layer 104 b is formed under P-well layer 102 withformation of input protective element M105, and the P-well layer 102 isisolated electrically from p type substrate 101;

[0055]FIG. 26 is a device sectional view of a modification of FIG. 25,where n type buried layer 104 c is formed so that the P-well layer 102with formation of the input protective element M105 is isolatedelectrically from the p type substrate 101;

[0056]FIG. 27 is a device sectional view of a modification of FIG. 25and FIG. 26, where P-well layer 103 with formation of memory array 1120is isolated electrically from the p type substrate 101 by first minoritycarrier absorbing area 113 formed to surround memory array and N typearea 104 d;

[0057] FIGS. 28(a) and (b) are device sectional views of a CMOS memoryin an applied state of the invention;

[0058]FIG. 29 is a sectional view showing a prior art;

[0059]FIG. 30 is a sectional view showing a first embodiment of a thirdaspect of the present invention;

[0060]FIG. 31 is a sectional view showing a second embodiment of thisthird aspect of the present invention; FIG.

[0061]FIG. 32 is a sectional view showing a third embodiment of thisthird aspect of the present invention;

[0062]FIG. 33 is a sectional view showing an embodiment of a CMOSstructure according to this third aspect of the present invention;

[0063]FIG. 34 is a-sectional view showing an embodiment of a bipolarstructure according to this third aspect of the present invention;

[0064]FIG. 35 is a sectional view showing an embodiment, according tothis third aspect, of a bipolar CMOS complex structure;

[0065]FIG. 36 is a sectional view showing an embodiment, according tothis third aspect, of an SOI structure;

[0066]FIG. 37 is a sectional view showing an embodiment of an SOSstructure in this third aspect of the present invention;

[0067]FIG. 38 is a block diagram of a memory;

[0068]FIG. 39 is a diagram showing an embodiment for the application ofa substrate isolation voltage to a memory;

[0069] FIGS. 40-44 are sectional views each showing an embodiment of aMOS dynamic memory;

[0070]FIGS. 45 and 46 are sectional views each showing an embodiment ofa bipolar CMOS complex dynamic memory;

[0071]FIG. 47 illustrates sectional views showing an embodiment ofprincipal steps for realizing the structure in FIG. 46;

[0072]FIG. 48 is a sectional view showing another embodiment, accordingto this third aspect of the present invention, of the bipolar-CMOScomplex dynamic memory;

[0073]FIG. 49 is a plan view with the embodiment of FIG. 48 seen fromthe front surface of a chip; and

[0074]FIG. 50 illustrates sectional views showing an embodiment ofprincipal steps for realizing the structure in FIG. 49.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0075] A DRAM having bipolar transistors which is an embodiment of thefirst aspect of the present invention is illustrated in FIG. 1(sectional view).

[0076] Referring to the figure, numeral 1 designates a p⁻-typesemiconductor substrate, and numeral 2 an n⁻-type epitaxial layer whichis stacked on the main surface of the semiconductor substrate 1. In thisembodiment, the semiconductor substrate 1 and the epitaxial layer 2function as a substantial semiconductor substrate and construct asemiconductor body. The conductivity types as specified above areselected for realizing npn-type bipolar transistors of high performancewhich are electrically isolated from one another. The impurityconcentration of the substrate 1 is selected on the order of10¹³-10¹⁷/cm⁻³ in consideration of the collector junction capacitance ofthe bipolar transistor, etc. The epitaxial layer 2 constructs a part ofan n-type well region for forming a P-channel MOSFET Q_(p), and a partof the collector of the bipolar transistor. The impurity concentrationof the epitaxial layer 2 is set according to the desired characteristicsof the respective transistors, and is selected on the order of10¹⁵-10¹⁷/cm³.

[0077] A minus potential of −2.5-−3.5 V is applied to the semiconductorsubstrate 1 by way of example for the purposes of preventing amalfunction ascribable to “undershoot” and reducing a pn-junctioncapacitance. The minus potential is supplied from-a built-in substratebias voltage generator or an external terminal to the semiconductorsubstrate 1.

[0078] An isolation region I for electrically isolating semiconductorelements (or circuit elements) is constructed of the semiconductorsubstrate 1, a p⁺-type buried semiconductor region 3, a p-typesemiconductor region 5 and a field insulator film 6. The p⁺-type buriedsemiconductor region 3 and the p-type semiconductor region 5 incombination can act to shield the memory cell from minority carriers, asdiscussed further infra.

[0079] The buried layer 3 is interposed between the semiconductorsubstrate 1 and the epitaxial layer 2. The semiconductor region 5 isdisposed in the main surface part of the epitaxial layer 2 overlying theburied layer 3. The field insulator film 6 is disposed on thesemiconductor region 5 and is made of, for example, a silicon oxide filmformed by the local thermal oxidation of the main surface of theepitaxial layer 2. The semiconductor region 5 is formed by the ionimplantation of boron in which an oxidation-impermeable film (siliconnitride film) for forming the field insulator film 6 is employed as amask, and the subsequent annealing for oxidation. The impurityconcentrations of the p⁺-type region 3 and the p-type region 5 are seton the order of 10¹⁶-10¹⁹/cm³ so that the individual circuit elementsmay be effectively isolated.

[0080] The impurity concentration of the p⁺-type buried layer 3 is alsoselected in consideration of the lowering of the resistance of a p-typewell region (to be described later) for an N-channel MOSFET and thereduction of soft errors in the N-channel MOSFET and a memory cell.

[0081] As shown on the left side in the figure, the bipolar transistorTr is constructed of a collector region (C) which is composed of ann⁺-type buried semiconductor region 4, an n⁺-type semiconductor region 8and the n-type epitaxial layer 2; a p-type base region (B) 9; and ann⁺-type emitter region (E) 16A. This bipolar transistor Tr is, ineffect, constructed of a known vertical structure of the npn-type.

[0082] The n⁺-type buried layer 4 is interposed between thesemiconductor substrate 1 and the epitaxial layer 2 in self-alignment tothe p⁺-type buried layer 3. This n⁺-type buried layer 4 lowers thecollector resistance to achieve a bipolar transistor of highperformance, and simultaneously serves to lower the resistance of ann-type well region (to be described later), and the impurityconcentration thereof is selected on the order of 10¹⁷-10²⁰/cm³. Then⁺-type buried layer 8 is made larger as compared with the emitter andbase regions in order to provide the collector electrode of the verticalnpn-type bipolar transistor of high performance in the main surface ofthe substrate (the semiconductor body).

[0083] The n⁺-type region 8 serves to connect the collector electrodeand the n⁺-type buried layer 4, and has its impurity concentration seton the order of 10¹⁷-10²¹/cm³ to the purpose of lowering the-collectorresistance.

[0084] The p-type base region 9 is formed in self-alignment to the fieldinsulator film 6 in a predetermined region within the n⁻-type epitaxiallayer 2 forming the collector of the bipolar-transistor, and has itsimpurity concentration set on the order of 10¹⁶-10¹⁸/cm³.

[0085] The emitter region 16A is constructed in such a way that theimpurity (for example, phosphorus or arsenic) of an electrode for anemitter, for example, polycrystalline silicon film 16 is diffused intothe base region 9 through a contact hole provided in an insulator film15. The impurity concentration of the emitter region 16A is set on theorder of 10¹⁷-10²⁰/cm³. Alternatively, the emitter forming method may besuch that the n⁺-type emitter region 16A is formed by diffusion from avapor phase or by ion implantation, and that a conductive material suchas aluminum is brought into electrical contact with the region 16A as anemitter electrode 22 directly through the intervention of a barriermetal such as platinum silicide. The semiconductor region 8 (collectorregion), the base region 9 and the emitter electrode 16 are respectivelyconnected to the collector electrode 22, a base electrode 22 and theemitter electrode 22 through contact holes 21 provided in an inter-layerinsulator film 20.

[0086] In order to shorten the access time of the DRAM, the bipolartransistor Tr constitutes a timing generator, an address buffer circuit,an address decoder circuit, a data input/output circuit, a mainamplifier or the like along with a CMOS circuit. Moreover, in theaddress buffer circuit and the data input/output circuit, the bipolartransistors facilitate the input and output operations of signals withTTL (Transistor-Transistor Logic) or ECL (Emitter Coupled Logic) levels.In particular, since the bipolar transistor Tr is a verticalnpn-transistor of high performance, it can readily construct an ECL typedifferential amplifier. By applying this amplifier to the input oroutput circuit, ECL signals of small logic amplitudes can be achieved athigh speed and at high reliability. In the data output circuit, thedrivability of an external device is enhanced. In the address decodercircuit, especially the signal level of a word line to which a greatcapacitive load is parasitic can be raised at high speed by driving theword line by means of the bipolar transistor Tr.

[0087] As shown in the middle part of the figure, the N-channel MOSFETQ_(n) constituting the peripheral circuit of the DRAM is constructedusing the p-type well region composed of the buried semiconductor region3 and a p⁻-type semiconductor region 7, and is composed of a gateinsulator film 15, a gate electrode 16, and a source region and a drainregion each of which is composed of a pair of semiconductor regions 17and 18 of the n-type and n⁺-type respectively.

[0088] The p⁺-type buried layer 3 is formed in order to help preventsoft errors from developing in the MOSFET Q_(n) (as will be describedlater). This is effective especially in the MOSFET Q_(n) forconstructing a sense amplifier. Moreover, since the p⁺-type buried layer3 lowers the resistance of the p-type well, it is effective forpreventing the occurrence of latch-up. The latch-up phenomenon isdetailed in “Technical Digest of International Electron Device Meeting,”1982, pp. 454-477, etc. Furthermore, the existence of the p⁺-type buriedlayer 3 makes it easy to turn the overlying n-type epitaxial layer 2into the p⁻-type semiconductor region 7 (having an impurityconcentration on the order of 10¹⁵-10¹⁷/cm³). As stated before, theimpurity concentration of the p⁺-type buried layer 3 is set on the orderof 10¹⁶-10¹⁹/cm³.

[0089] The same potential as that of the substrate 1 is applied to thep-type well region. Though not shown, a wiring lead which is made thesame layer as the electrode 22 and to which the substrate potential isapplied is connected to a p⁺-type region which is formed within thep-type well by the same step as that of p⁺-type regions 19 to bedescribed later.

[0090] The n-type semiconductor regions 17 are interposed between then⁺-type regions 18 and a channel forming region, and constitute theMOSFET of the LDD (Lightly Doped Drain) structure stated in “IEEETransactions on Electron Devices,” Vol. ED-27, pp. 1359-1367, August1980. The n-type regions 17 are formed by, e.g., ion implantationemploying the gate electrode 16 as a mask and have their impurityconcentration set on the order of 10¹⁵-10¹⁷/cm The n⁺-type regions 18are formed by, e.g., ion implantation employing as a mask the gateelectrode 16 and side-wall insulator films 23 which are formed on thesides of the gate electrode 16 in self-alignment thereto, and theirimpurity concentration is set on the order of 10¹⁷-10²¹/cm³.

[0091] As shown at the middle part in the figure, the P-channel MOSFETQ_(p) constituting the peripheral circuit of the DRAM is constructedusing the n-type well region composed of the buried semiconductor region4 and the epitaxial layer 2, and is composed of a gate insulator film15, a gate electrode 16, and the p⁺-type source and drain regions 19.

[0092] Likewise to the p⁺-type buried layer 3, the n⁺-type buried layer4 lowers the resistance of the n-type well region and is thereforeeffective for the prevention of latch-up.

[0093] A power source potential V_(cc) is applied to the n-type wellregion. Though not shown, a wiring lead which is made of the same layeras the electrode 22 and to which the power source potential is appliedis connected to an n⁺-type region which is formed within the n-type wellby the same step as that of the n⁺-type regions 18.

[0094] Electrodes 22 made of aluminum are connected to the source anddrain regions of the MOSFETs Q_(n) and Q_(p) through contact holes whichare formed in the insulator film 15 and the inter-layer insulator film20 formed of, e.g., a PSG (phophosilicate glass) film on the wholesurface of the substrate.

[0095] The gate-electrode 16 is made of a polycrystalline silicon-film.In this embodiment, the gate electrode 16 is formed by the same step asthat of the emitter electrode 16. After the gate insulator film 15 hasbeen formed, it is removed-from the predetermined area for forming theemitter. In the predetermined area, from the polycrystalline siliconfilm 16 connected to the main surface of the substrate (epitaxial layer2), an impurity is diffused into the base region 9, whereby the emitterregion is formed.

[0096] The electrode 16 may well be made of a polycrystalline siliconfilm on which a film of a refractory metal (such as molybdenum,tungsten, titanium or tantalum) or a film of a silicide thereof isstacked.

[0097] In a case where the gate electrode 16 is formed by a stepseparate from that of the emitter electrode 16, it may well be a singlelayer of the refractory metal film or the silicide film mentioned above.

[0098] As shown on the right side in FIG. 1, the memory cell M of theDRAM is constructed of a series circuit which comprises a switching(memory cell selecting) N-channel MOSFET Q_(s) and a capacitor C_(p).This memory cell is formed in the p-type well region which is composedof the buried semiconductor region 3 and the semiconductor region 7.

[0099] The capacitor C_(p) is chiefly constructed of a MIS typecapacitance element which is configured of n-type semiconductor regions12, a dielectric film 11 and a plate electrode 13, and to which apn-junction capacitance element composed of the semiconductor regions 12and a p⁺-type semiconductor region 10 is added. The impurityconcentration of the n-type semiconductor regions 12 forming oneelectrode of the capacitor C_(p) is set on the order of 10¹⁷-10²¹/cm³.The dielectric film 11 is made of, for example, a triple film whichcomprises a silicon oxide film formed by the thermal oxidation of thesubstrate, a silicon nitride film formed by CVD, and a silicon oxidefilm formed by the thermal oxidation of the silicon nitride film. Theplate electrode 13 forming the other electrode of the capacitor C_(p) ismade of a polycrystalline silicon film which has its resistance loweredby introducing phosphorus, and it is a flat electrode which is common tothe plurality of memory cells of an identical (single) memory cellarray. The impurity concentration of the p⁺-type semiconductor region 10is set on the order of 10¹⁶-10¹⁹ cm³. The p⁺-type region 10 is formed inorder to reduce soft errors in the memory cell. That is, the p⁺-typeregion is formed in order to increase the capacitance of the capacitor Cand to form a potential barrier to minority carriers. A potential (forexample, 5 V=V_(cc) for a high level or 0 V=V_(ss) for a low level)corresponding to information “0” or “1” transmitted from a data line DL(the aluminum wiring layer 22) through the MOSFET is applied to thesemiconductor regions 12. By way of examples a potential (½ V_(cc)≈2.5V) intermediate between the information “0” and the information “1” isapplied to the plate electrode 13.

[0100] An insulator film 14 is formed so as to cover the plate electrode13, and effects the electrical isolation between this plate electrode 13and a word line (WL) 16, WL extending over it. An insulator film 11A isformed so as to electrically isolate the capacitors C_(p), along withthe semiconductor region 10.

[0101] Likewise to the foregoing MOSFET Q_(n), the MOSFET Q_(s) isconstructed of a gate insulator film 15, a gate electrode 16, a pair ofsemiconductor regions 17, and source and drain regions 18.

[0102] One of the source and drain regions 18 of the MOSFET Q_(s) iselectrically connected with the data line (DL) 22,DL.

[0103] At a position close to the bipolar transistor Tr there isarranged an n⁺-type semiconductor region (not shown) functioning as aninjection source which injects minority carriers into the semiconductorsubstrate 1 owing to the operation of a parasitic bipolar transistor.This semiconductor region is, for example, a wiring layer or the sourceregion or drain region of an N-channel MISFET. The parasitic bipolartransistor is constructed with its base region being the collectorregion composed of the buried layer 4 and the semiconductor region 8,its emitter region being the base region 9, and its collector regionbeing the semiconductor substrate 1.

[0104] On the other hand, a buried layer 3 which has the sameconductivity type as that of the semiconductor substrate 1 (or thesemiconductor region 7) and an impurity concentration higher than thatof the same is interposed between the semiconductor substrate 1 and theepitaxial layer 2 underlying the memory cell. Thus, a potential barriercan be formed against the minority carriers which are injected due tothe operation of the parasitic bipolar transistor and toward thesemiconductor substrate 1 from the n⁺-type semiconductor region arrangednearby, and against minority carriers which are created within thesemiconductor substrate 1 underneath the MOSFET Q_(s) or the capacitorC_(p) by alpha particles. Accordingly, the minority carriers can beprevented from entering the memory cell. In addition, when an electricfield is applied to any of the n-type regions (source and drain regions,etc.) of the N-channel MOSFET etc. in the memory cell, a depletion layerspreads in the p-type well layer 7. As the region of this depletionlayer spreads more, the electrons created by the alpha particles arecollected more. In the case where the p⁺-type region 3 underlies thememory cell as in the present invention, the stretch of the depletionlayer is checked by this p⁺-type region 3. Even when a voltage isapplied, the depletion region does not spread beyond the p⁺-type region3. Thus, the alpha particle-immunity of the DRAM can be enhanced. Thatis, the access time can be shortened, while at the same time the softerrors can be prevented to enhance the electrical reliability.

[0105] Besides, the buried semiconductor region 3 under the memory cellcan be formed by the same manufacturing step as that of the buriedsemiconductor region 3 constructing the p-type well region of the MOSFETQ_(n) and the buried semiconductor region 3 constructing the isolationregion I. In other words, a separate manufacturing step for expresslyforming the buried semiconductor region 3 under the memory cell can beavoided.

[0106] In the same manner as described above, soft errors in theN-channel MOSFET Q_(n) of the peripheral circuit (especially, the senseamplifier) can be prevented. In a case where data read out from thememory cell to the data-line DL has been supplied to the n-typesemiconductor region of the MOSFET Q_(n) connected to the data line DL,the inversion of the data in this region can be prevented.

[0107] Moreover, the existence of the n⁺-type buried layer 4 preventsthe soft errors of the P-channel MOSFET Q_(p) of the peripheral circuitattributed to holes.

[0108] Excepting the regions 2, 3, 4, 5 and 7, the memory cell M and theP- and N-channel MOSFETs Q_(p) and Q_(n) of the DRAM in FIG. 1 aresubstantially the same as those of a DRAM disclosed in U.S. patentapplication Ser. No. 855,418 filed on Apr. 24, 1986, and the contentsthereof are incorporated herein by reference. Throughout thisspecification, the features of the above patent application cited as areference shall be referred to as those of the present invention.

[0109] The second embodiment of the present invention comprises a DRAMwhich attains a rise in the operating speed of a peripheral circuitparticularly in case of supplying a minus potential to a substrate. TheDRAM being the second embodiment is illustrated in FIG. 2.

[0110] In the description of the second embodiment (et seq.), only thepoints of difference from the first embodiment will be elucidated.

[0111] As seen from FIG. 2, the DRAM of the second embodiment is notprovided with the p⁺-type buried layer 3 in the region for forming theN-channel MOSFET Q_(n) constituting the peripheral circuit. Since thesemiconductor substrate 1 is held at the minus potential, a depletionlayer formed in the channel forming region of the MOSFET Q_(n) isstretched deeper in the depth direction of the-semiconductor substrate 1(it does not touch the buried semiconductor region 3). For this reason,the fluctuation of the threshold voltage of the MOSFET Q_(n) attributedto the fluctuation of the substrate potential can be lessened, that is,the substrate effect constant can be made smaller. Owing to the lessfluctuation of the threshold voltage, the value of the threshold voltagein the ordinary state can be made small. In a case where the thresholdvoltage fluctuates greatly, the fluctuation in the minus directionrenders the MOSFET “normally-on” and incurs a malfunction. After all, itis possible to reduce an impurity concentration within the depletionlayer of the channel forming region and to lower the threshold voltageof the MOSFET Q_(n). Owing to the lowering of the threshold voltage ofthe MOSFET Q_(n) a raised switching speed can be attained, with theresult that the operating speed of the peripheral circuit can be raisedwhile substantially the same effects as those of the first embodimentare produced.

[0112] As illustrated in FIG. 3, the third embodiment of the presentinvention is an example of a DRAM in which, contrariwise to the secondembodiment, the p⁺-type buried layer 3 is provided under the N-channelMOSFET Q_(n) of the peripheral circuit and is not provided under thememory cell portion M.

[0113] The present embodiment has been realized with note taken of thefact that the probability at which minority carriers (electrons) formingthe cause of soft errors are trapped is proportional to the area of(n⁺-type) semiconductor regions constituting a circuit element. Morespecifically, the area of the n⁺-type source and drain regions of theN-channel MOSFETs Q_(n) of the peripheral circuit, especially one senseamplifier, is much larger than that of the n⁺-type semiconductor regionswithin one memory cell. Thus, according to the present embodiment, theinvasion of the minority carriers into the MOSFETs Q_(n) of theperipheral circuit liable to the soft errors can be prevented by thep⁺-type buried layer 3, so that the soft errors in the peripheralcircuit can be prevented.

[0114] In each of the DRAMs shown in FIGS. 2 and 3, the formation of thep⁺-type region 10 can be omitted.

[0115]FIG. 4 is a graph showing improvements in the soft error ratewhich are attained by the present invention.

[0116] In FIG. 4, both the axis of ordinates and the axis of abscissasare graduated logarithmically. The axis of abscissas represents theoperating cycle time of the DRAM, in other words, the interval of thefalling edges of row address strobe signals {overscore (RAS)} in thecase where data is repeatedly read or written. The axis of ordinatesrepresents the rate of occurrence of soft errors in a data line mode.The soft error rate is indicated by relative values with a predeterminedvalue set at unity (a reference value).

[0117] Straight lines A, B and C denote the soft error rates of theDRAMs in FIGS. 1, 2 and 3, respectively. A straight line D denotes thesoft error rate of a DRAM which is not formed with the p⁺-type buriedsemiconductor regions 3 in FIGS. 1-3.

[0118] In the DRAM having bipolar transistors, the soft error rate isimproved in the case (straight line B) where the p⁺-type buried layer 3is disposed under the memory cell portion M according to the presentinvention, as compared with the case (straight line D) where it is notformed. This is because the minority carriers can be prevented fromentering the n-type region 12 of the capacitor C_(p) and the n-typesource and drain regions 17 and 18 of the MOSFET Q_(s), namely, thesemiconductor regions which are directly or indirectly coupled(connected) to the data line 22.

[0119] The soft error rate (straight line C) of the DRAM in FIG. 3 islower than those of the DRAMs of the straight lines B and D. The area ofthe n⁺-type semiconductor regions of the MOSFETs of the sense amplifieris larger than that of the n⁺-type semiconductor regions within oneselected memory cell. Among the peripheral circuits, principally thesense amplifier has the soft errors prevented, so that the soft errorrate of the DRAM is improved.

[0120] The soft error rate (straight line A) of the DRAM in FIG. 1 isthe most excellent. Herein, the soft error rate is improved in excess ofthe sum of the improvements in the soft error rates indicated at thestraight lines B and C.

[0121] As the operating cycle time of the DRAM becomes longer, the softerrors in the data line mode decrease more. This is because the chancesof trapping the minority carriers lessen outside the capacitor C_(p) ofthe memory cell. Accordingly, the soft errors in the data line mode needto be reduced for the purpose of raising the operating speed of theDRAM. The present invention is effective for attaining the raised speedof the DRAM from this viewpoint, in addition to the use of the bipolartransistors.

[0122] In the first-to third embodiments, the n-type well region forforming the P-channel MOSFET Q_(p) may well be formed as shown in FIG.5.

[0123] Referring to FIG. 5, a potential (the power source potentialV_(cc)) to the n-type well region is supplied through an n⁺-type region8A which is deeper than the n⁺-type region 18. The n⁺-type region 8A isformed by the same step as that of the n⁺-type region 8 which is thecollector of the bipolar transistor. It is accordingly formed so as tolie in contact with the n⁺-type buried layer 4 of the n-type wellregion. Thus, the resistance of the n-type well region can be renderedstill lower, and the occurrence of the latch-up phenomenon can bechecked.

[0124] The construction of the memory cell may well be any ofconstructions illustrated in FIGS. 6-10. Each of these-figures showsonly the memory cell portion M and the N-channel MOSFET Q_(n) of theperipheral circuit.

[0125] The memory cell in FIG. 6 is of the planar type similarly tothose in FIGS. 1-3, but the capacitors C_(p) of the adjacent memorycells are isolated by the field insulator film 6 and the p-typesemiconductor region 5. This memory cell in FIG. 6 is an example inwhich the present invention is applied to a memory cell described in“1977 International Electron Devices Meeting, Technical Digest,” pp.287-290. The MOSFET Q_(s) does not have the side-wall insulator films23, and accordingly has the single-drain structure in which the sourceand drain regions are made up of only the n⁺-type regions 18. Likewise,the MOSFET Q_(n) is put into the single-drain structure.

[0126] In the memory cell of FIG. 6, the p⁺-type region 10 or both then⁺-type and p⁺-type regions 12 and 10 may well be omitted. Here, whenboth the n⁺-type and p⁺-type regions 12 and 10 are omitted, thepotential of the plate electrode 13 is set at the power source potentialV_(cc).

[0127] The memory cell in FIG. 7 is so configured that the capacitorC_(p) is stacked on the main surface of the substrate. The capacitorC_(p) is formed between an electrode 26 and an electrode 24A which isconnected to one of the n-type regions (each being composed of the pairof regions 17 and 18) of a MOSFET and which is led out on the insulatorfilm 6 for element isolation. The electrodes 24A and 26 are chieflyformed of polycrystalline silicon or the like. An insulator film 25 isthe dielectric film of the capacitor, and is formed of a materialsimilar to that of the insulator film 11. Shown at numeral 27 is aninter-layer insulator film.

[0128] In the construction of FIG. 7, the wiring layer 22 and theother-pair of n-type regions 17 and 18 are connected through anelectrode 24B which is formed simultaneously with the electrode 24A.

[0129] According to the construction of FIG. 7, the capacitor C_(p) isformed in isolation from the silicon substrate, and hence, electrons donot gather in the capacitor portion to incur malfunctions. Such a memorycell is described in, for example, “IEEE Journal of Solid StateCircuits,” Vol. SC-15, No. 4, August 1980, pp. 661-667 or “InternationalSolid-State Circuits Conference (ISSCC), Digest of Technical Papers,”February 1985, pp. 250-251.

[0130] In the memory cell of FIG. 7, electrons created within thesubstrate are prevented from collecting into the n⁺-type region 18underneath the capacitor C_(p). Thus, according to the presentembodiment, the effect based on the isolation of the capacitor from thesilicon substrate and the effect of the present invention contributesynergistically to raising the immunity against the soft errors stillmore.

[0131] The connection between the source or drain regions 18 of theMOSFET Q_(n) and the electrode 22 (not shown) thereof may well beexecuted through the polycrystalline silicon film 24B as in the memorycell portion.

[0132] In the embodiment of FIG. 7, p⁺-type semiconductor regions 28 areprovided under the n-type regions 18 (and/or 17) of the MOSFETs Q_(n)and Q_(s), to form potential barriers also in these parts. The regions28 are set at an impurity concentration nearly equal to that of theregion 10. The potential barriers are formed double by the regions 3 and28, and the effect of the improvement in the soft errors becomes verygreat.

[0133] The measure of underlaying the n-type impurity layers with thep⁺-type regions 28 as in the present embodiment can be similarly appliedto all the other embodiments. The p⁺-type regions 28 can also be formedeither under the MOSFET Q_(s) of the memory cell or under the MOSFETQ_(n) of the peripheral circuit. The formation of the p⁺-type regions 28may well be omitted. Formation of the p⁺-type region 3, or both thep⁺-type regions 3 and 28 of the memory cell, in FIG. 7, can also beomitted.

[0134] The MOSFET Q_(s) may well have the single-drain structure. Onthis occasion, the MOSFET Q_(n) may be put into either the single-drainstructure or the LDD structure.

[0135]FIG. 8 shows an example in which the p⁺-type regions 28 statedabove are formed in a case where, as in the DRAM of FIG. 2, the p⁺-typeburied layer 3 does not exist under the MOSFET Q_(n) (especially, underthe n⁺-type regions 18) of the peripheral circuit. That is, this exampleavoids the rise of the threshold voltage attributed to the p⁺-typeburied layer 3 and also improves the soft error rate by means of thep⁺-type regions 28.

[0136] According to the present embodiment, problems which might becaused by the provision of the buried layer can be solved. Morespecifically, in a case where the circuit performance changesdrastically when, for example, the impurity of the buried layer 3reaches the vicinity of the-n-type regions 18 (and 17) or the gate ofthe MOSFET to lower the junction breakdown voltage though slightly-or toraise the threshold voltage of the MOSFET though slightly, the buriedlayer 3 is not provided in the corresponding part only, whereby such aproblem can be solved.

[0137]FIG. 9 shows an example in which the impurity concentration of theburied layer 3 is changed in selected parts, and the p-type buried layer3 is endowed with different concentrations under the memory cell andunder the MOSFET Q_(n) of the peripheral circuit. By way of example, theimpurity concentration of the p⁺-type buried layer 3A under the MOSFETQ_(n) is set lower than that of the p⁺-type buried layer 3, and higherthan that of the substrate 1 and the region 7, in order to make the riseof the threshold voltage small. According to the present embodiment, theimpurity concentrations can be set for the individual parts, so that amemory of higher performance as compared with the example of FIG. 7,with dispersions in the soft error characteristic and other electricalcharacteristics taken into consideration, can be realized.

[0138]FIG. 10 shows an example in which the capacitor C_(p) is formed byutilizing a trench or moat 29 extended in the depth direction from themain surface of the semiconductor substrate (body).

[0139] The capacitor C_(p) is constructed of a polycrystalline siliconfilm 30 being one electrode, the dielectric film 11, and thesemiconductor body being the other electrode. Unlike the electrode 13,the electrode 30 is formed independently for each memory cell and isconnected to the n⁺-type region 18 of the MOSFET Q_(s). Thesemiconductor body is used as the electrode common to all the memorycells, and has a fixed potential (for example, the ground potentialV_(SS) of the circuitry, or the minus substrate bias potential V_(BB))applied thereto. Electrons created within the substrate 1 by the bipolartransistor Tr are prevented from entering the memory cell by the p⁺-typeburied layer 3. That is, the p⁺-type buried layer 3 and upper parts(shallower parts) can be utilized for the capacitor C_(p) of slight softerrors.

[0140] The DRAM in FIG. 1 can be fabricated by combining methods ofproducing semiconductor integrated circuit devices as disclosed in U.S.patent application Ser. No. 554,794 filed on Nov. 23, 1983 and U.S.patent application Ser. No. 855,418 filed on Apr. 24, 1986, the contentsof each of which are incorporated herein by reference. Morespecifically, the steps of forming the regions 2, 3, 4, 5 and 7 and theinsulator film 6 on the semiconductor substrate 1 conform to the patentapplication Ser. No. 554,794. The step of forming the bipolar transistorTr is also taught in this patent application. On the other hand, thesteps of forming the memory cell M and the MOSFETs Q_(n) and Q_(p) aretaught in the patent application Ser. No. 855,418. That is, after thesemiconductor body has been formed in conformity with the former, thebipolar transistor Tr conforming to the former and the memory cell M andMOSFETs Q_(n) and Q_(p) conforming to the latter are formed in the body.

[0141] The DRAM in FIG. 2 or FIG. 3 can be formed in such a way that,when the impurity for the p⁺-type region 3 is to be introduced into thesubstrate 1, the region Q_(n) or the region M is selectively coveredwith a mask of photoresist or the like.

[0142] To sum up, according to this first aspect of the presentinvention, it becomes possible to form a DRAM having bipolartransistors. The bipolar transistors are caused to coexist in the DRAM,whereby the operating speed of the DRAM can be raised, and at the sametime, potential barriers are formed against minority carriers created bythe bipolar transistors, whereby soft errors ascribable to the minoritycarriers can be prevented to enhance the electrical reliability of theDRAM.

[0143] Although, in the above, the invention made by the inventors hasbeen concretely described in conjunction with embodiments, it is amatter of course that the present invention is not restricted to theforegoing embodiments but that it can be variously modified within ascope not departing from the purport thereof.

[0144] The bipolar transistor can adopt various structures.

[0145] As shown in FIG. 11 or FIG. 12 by way of example, the bipolartransistor may well have a structure which includes a collector formedof an n-type well region 31, a p-type base region 32, and an n⁺-typeemitter region 18A. An n⁺-type semiconductor region 18B and a p⁺-typesemiconductor region 19A are regions for connecting electrodes (notshown) made of aluminum, and are respectively formed by the same stepsas those of the source and drain regions of the N-channel and P-channelMOSFETs. The fact that the collector 31 is provided with theheavily-doped layers 18B in two places, is intended to reduce theresistance of the collector 31, thereby to prevent the bipolartransistor from being saturated due to the potential of the collectorbeing lowered when current flows to this collector. Needless to say, ifnecessary, only one of the layers 18B may well be provided, or theresistance of the collector 31 may well be lowered still more with theheavily-doped layer 18B encircling the base 32. The lower resistance canalso be attained in such a way that an n-type layer which is higher inthe impurity concentration than the region 31 is interposed between theregion 31 and the substrate 1.

[0146] By simplifying the structure of the bipolar transistor, the stepsof forming the p-type and n-type buried layers, the epitaxial layer,etc. are dispensed with. That is, the manufacturing steps decrease innumber and become simpler. The performance of this bipolar transistor issomewhat inferior to that of the bipolar transistor in FIG. 1.

[0147] A memory cell in FIG. 11 is the same as the memory cell shown inFIG. 6.

[0148] As will be discussed later, a p⁺-type region 32 for preventingsoft errors in the memory cell is formed by the same step as that of thebase region 32 of the bipolar transistor though this is not especiallyrestrictive. The impurity concentration of the p⁺-type region 32 is seton the order of 10¹⁶-10¹⁹/cm³.

[0149] In FIG. 11 et seq., insulator films, wiring etc. on the substrateare not shown.

[0150] It is also possible to form the p⁺-type buried layer 3 in thestructure of FIG. 11. A memory cell and an N-channel MOSFET constitutinga peripheral circuit in FIG. 12 are substantially the same as those inFIG. 9.

[0151] In the DRAM of FIG. 12, it is also possible to omit only p⁺-typeregions 28 under the memory cell.

[0152] According to this embodiment, the DRAM having the bipolartransistors can be produced by that number of steps which is close tothe number of manufacturing steps of a CMOS circuit. By taking thestructure of FIG. 11 as an example, a manufacturing method will bebriefly exemplified with reference to FIGS. 13A-13D.

[0153] As shown in FIG. 13A, a silicon substrate 1A doped with a p-typeimpurity such as boron is prepared. The concentration of the impurity isusually set within a range on the order of 10¹³-10¹⁷/cm³. Subsequently,n-type regions (n-type wells) 31 are formed in the main surface of thesilicon substrate 1A by ion implantation or conventional diffusion. Atthe next step, an insulator film 6 made of SiO₂ is formed by a knownLOCOS (Local Oxidation of Silicon) process.

[0154] As shown in FIG. 13B, a p-type layer 32 to form a barrier and ap-type layer 32 to serve as the base of a bipolar transistor aresimultaneously formed by the conventional diffusion or the ionimplantation. Subsequently, an n-type layer 12 to serve as one electrodeof a capacitor is formed.

[0155] As shown in FIG. 13C, the insulator film 11 of the capacitorC_(p) is formed by the oxidation of the surface of the silicon substrate1A, and an electrode 13 is formed thereon. The material of the electrode13 is, for example, polycrystalline silicon. Next, the gate insulatorfilm 15 of a MOSFET Q_(s) is formed by the oxidation of the surface ofthe silicon substrate 1A, and the gate electrode 16 thereof may beformed on the gate insulator film. Here, the insulator film 15 and theelectrode 16 may be deposited on the whole surface of the siliconsubstrate 1A in a stacked state and be thereafter processed by knownphotoetching at the same time.

[0156] Subsequently, as shown in FIG. 13D, n⁺-type regions 18 to serveas the source and drain of an N-channel MOSFET, and an n⁺-type region18A forming the emitter of the bipolar transistor and n⁺-type regions18B constituting the collector portion thereof are simultaneously formedby ion implantation.

[0157] Thereafter, p⁺-type regions 19 to serve as the source and drainof a P-channel MOSFET and a p⁺-type region 19A within the base of thebipolar transistor are simultaneously formed by ion implantation. Then,the structure depicted in FIG. 11 is obtained. Although insulator filmson the gate electrodes of the MOSFETs, wiring lines such as data lines,etc. have been omitted here, they can be readily formed by known steps.

[0158] According to the manufacturing method described above, the p-typelayer functioning as the barrier and the p-type region 32 functioning asthe base of the bipolar transistor can be formed by the identical step,and also the n-type well 31 for the P-channel MOSFET and the collector31 of the bipolar transistor can be formed by an identical (single)step. Further, the n⁺-type regions 18 to serve as the source and drainof the N-channel MOSFET, and the n⁺-type region 18A forming the emitterof the bipolar transistor and the n⁺-type regions 18B constituting thecollector portion thereof can be formed by an identical (single) step.Moreover, the p-type layers 19 serving as the source and drain of theP-channel MOSFET and the p⁺-type region 19A within the base of thebipolar transistor can be formed by an identical (single) step.

[0159] The bipolar transistor can adopt various configurations differentform the foregoing structure.

[0160] The circuit elements may well be isolated by the p⁻-typesemiconductor substrate 1 and the field insulator film 6 withoutproviding the p⁺-type buried semiconductor regions 3 and the p-typesemiconductor regions 5.

[0161] The peripheral circuits may well be constructed of N-channelMISFETs and bipolar transistors, not of the CMOS circuits.

[0162] Although the p-type buried layer 3 functioning as the potentialbarrier has been exemplified as being formed apart from the source anddrain electrodes, etc. of the MOSFET, it may well be formed in closeproximity thereto or in contact therewith in some cases.

[0163] The present invention is applicable, not only to the memoryemploying the 1-transistor and 1-capacitor type memory cell, but also tomemories employing, for example, a 3-transistor type memory celldescribed in “Electronics,” Feb. 16, 1970, pp. 109-115, etc. or a4-transistor type memory cell described in “Fall Joint ComputerConference in 1970, Collection of Papers,” pp. 54-62.

[0164] Before describing specific embodiments of the second aspect ofthe present invention, an outline of a Bi-CMOS static RAM (hereinafterreferred to as “Bi-CMOS SRAM”) with high speed and low power consumptionwill be first described; while not limited thereto, such Bi-CMOS SRAM isa type of device to which this second aspect of the present inventioncan advantageously be applied, and this second aspect of the presentinvention will be described in connection therewith.

[0165] Next, a detailed description of the novel information destructionmechanism clarified by the inventors, discussed briefly previously, willbe described, and then specific embodiments will be described.

[0166] a) Outline of Bi-CMOS SRAM

[0167] In the circuit constitution of a Bi-CMOS SRAM as disclosed inpreviously mentioned U.S. patent application Ser. No. 701,226, a Bi-CMOScomplex switching circuit is used in a peripheral circuit such as anaddress circuit or timing circuit, and a flip-flop type memory cell witha high resistance load is used in the memory cell.

[0168]FIG. 14 is a circuit diagram of a Bi-CMOS SRAM illustrating anexample of circuit constitution of word line driver circuits WD₁, WD₂ ofperipheral circuit 1110 and memory cells 1121, 1121.

[0169] As shown in FIG. 14, logical circuit (switching circuit) of theperipheral circuit unit 1110 comprises bipolar transistors Q₁, Q₂,P-channel MOS field effect transistor M1, N-channel MOS field effecttransistors M2, M3 and diode D₁. The bipolar transistors Q₁, Q₂ areconnected in totem pole connection, and constitute an output stage ofthe CMOS logical circuit (M1, M2). The MOS field effect transistors M1,M2 constitute a pre-stage drive circuit by CMOS. That is, the logicalcircuit (switching circuit) of the peripheral circuit unit 1110 includesthe bipolar transistors and the MOS field effect transistors. R1designates a series resistor in parasitic constitution as a device atthe collector side of the bipolar transistor Q₁. In FIG. 14, X₁-X₃designate internal address signals.

[0170] In the memory cell array unit 1120, a number of memory cells 1121are arranged in matrix form of rows and columns. Word line W forselecting a prescribed memory cell from a plurality of memory cells 1121and complementary data lines D, {overscore (D)} are connected to eachmemory cell 1121. As shown in FIG. 14, a plurality of word lines W₁, W₂,. . . and a plurality of complementary data lines D₁, {overscore (D₁)}(D₂, {overscore (D₂)}) are formed perpendicular to each other. Althoughnot shown, the complementary data lines D₁, {overscore (D₁)} areconnected through a column switch to a sense amplifier and outputcircuit. Each of the memory cells (MC11, MC12) 1121 includes a pair ofn-channel MOS field effect transistors M11, M12 with each input andoutput cross-coupled, load resistors R11, R12 made of polysilicon havinghigh resistance value in giga-hom unit and connected in series betweenthe output of the n-channel MOS field effect transistor and theoperation potential Vcc, and-MOS field-effect transistors M13, M14 astransfer switch connected between the complementary data line pair D₁,{overscore (D₁)} and output of the n-channel MOSFET M11, M12. That is,the memory cell 1121 constitutes a sort of flip-flop type holdingcircuit.

[0171] In FIG. 14, MOSFET M4 is a Bi-CMOS complex circuit formed in theperipheral circuit unit 1110 or N-channel MOSFET contained in the CMOScircuit. That is, the NMOSFET M4 represents transistor similar to theNMOSFET M3 of the word line driver circuit WD₂ contained in the linedriver circuit WD₂ contained in the line driver circuit WD₃ (WD₁).

[0172]FIG. 15 partially shows the layout arrangement of the peripheralcircuit unit 1110 and the memory cell array unit -1120. 20—As shownin-FIG. 2, the peripheral circuit unit 1110 is formed along thecircumference of the memory cell array unit 1120.

[0173] As above described, the logical circuit of a Bi-CMOS type is usedin the peripheral circuit unit 1110, whereby a static RAM can berealized having both high speed and low power consumption.

[0174] The word line drivers WD₁-WD₆ shown in FIG. 14, FIG. 15 areinverter circuits, and the transistors at the output stage are bipolar.This is shown by the black painting at output of the logical symbol ofthe inverter.

[0175]FIG. 16 shows a device sectional view of the bipolar transistor Q₁of the word line driver WD₂ in FIG. 15, the NMOSFET M4 with source atGND potential in another peripheral circuit, the NMOSFET M11 containedin the memory cell MC11, and the resistor R₁.

[0176] The Bi-CMOS static RAM 1100 shown in FIG. 16 is formed usingp-type monocrystalline semiconductor substrate 100. On the p⁻ typesemiconductor substrate 100 are formed p⁺ type buried layer 102, p typediffusion layer (well) 103, n⁺ type buried layer 104 and n type welldiffusion layer 105, and adoption of the double well system obviates thenecessity of an isolation layer.

[0177] In order to constitute n-channel MOS field effect transistors M4,M11 on the p type well diffusion layer 103, a gate electrode 108 made ofpolysilicon, n⁺ type source•drain diffusion layer 106 in self-aligningformation and p⁺ type well connecting diffusion layer 107 are formed.The-gate electrode 108 may be formed of conductive polysilicon,silicide, high melting point metal or by a combination of these. GNDdesignates the grounding potential being the operation potential at lowlevel.

[0178] In order to constitute bipolar transistor Q₁ on the n type welldiffusion layer 105, n⁺ type collector electrode drawing diffusion layer109, p type base diffusion layer 110, n⁺ type emitter diffusion layer111 and source•drain of p-channel MOSFET (not shown) are formed. Cdesignates the collector, E designates the emitter and B designates thebase. Numeral 112 designates an oxide film (SiO₂ film) to cover thesubstrate surface. The oxide film 112 is provided with an aperture fordrawing electrode, and an aluminium electrode Al as wiring is formed inthe aperture. The emitter electrode E and the source•drain electrode ofthe NMOSFET M4, M11 may be formed by conductive polysilicon.

[0179] The Bi-CMOS structure can be formed according to the followingprocess. The p type monocrystalline substrate 100 is prepared, and thesurface oxide film with film thickness of about 430 Å is formed on thesurface of the substrate 100. A nitride film (Si₃N₄) is formedselectively on the surface oxide film. Using the nitride film as animpurity introducing mask, antimony is introduced in the p typesubstrate 100 and the N type buried layer 104 is formed. Next, using thenitride film as an anti-oxidation mask, an oxide film with filmthickness of about 4000 Å is formed on the N type buried layer 104 andthen the nitride film is removed. Using the oxide film on the N typeburied layer 104 as an impurity introducing mask, boron is ion-implantedin self-alignment and the p type buried layer 102 is formed. The surfaceoxide film is entirely removed, and an n type epitaxial layer is formedon the substrate 100 and further SiO₂ film of about 950° C. is formed onthe surface of the epitaxial layer. And then masking and processingsimilar to the forming process of the N type and P type buried layersare performed thereby the N type well layer 105 and the P type welllayer 103 are formed. The N-channel MOSFET is formed in the P type welllayer 104, and the P-channel MOSFET and npn bipolar transistor areformed in the N type well layer 105. A feature of the manufacturingprocess is that adoption of the double well system obviates thenecessity of an isolation layer, and that the well layers 103, 105 andthe buried layers 102, 104 can be formed using-the same mask thereby themanufacturing masks may be reduced in number.

[0180] b) Novel Memory Cell Information Destruction Mechanism

[0181] I. As shown in FIG. 16, pnp type parasitic bipolar transistor Qs1and npn type parasitic bipolar transistor Qs2 are latently formed in thesemiconductor area extending from the peripheral circuit unit 1110 tothe memory cell array unit 1120. Further the two parasitic bipolartransistors Qs1, Qs2 are connected to each other through resistors Rs1,Rs2 being parasitic in the diffusion layer.

[0182] In FIG. 14, if the source voltage Vcc is large, transient-currentflowing instantaneously through the bipolar transistor Q₁ at outputstage increases during the switching of the CMOS type logic circuit. Ifthe transient current increases, the bipolar transistor is significantlysaturated instantaneously.

[0183] In FIG. 14 and FIG. 16, if the bipolar transistor Q₁ issignificantly saturated even instantaneously, the p type base area 110and the n type areas 104, 105 are biased forward. That is, the collectorpotential of the bipolar transistor Q₁ becomes lower than the basepotential. Then current I₁ flows from base B side of the bipolartransistor Q₁ to collector C side. Base•emitter voltage of the pnp typeparasitic bipolar transistor Qs1 is generated and therefore thetransistor Qs1 is rendered on. Thereby collector current I₂ flowsthrough the pnp type parasitic bipolar transistor Qs1. If the current I₂flows, the potential of the substrate 100 rises by the parasiticresistors Rs1, Rs2 in the p type area (100,102,103). Further, thecollector current I₂ supplies the npn type parasitic bipolar transistorQs2 with base current. Base•emitter voltage of the npn type parasiticbipolar transistor Qs2 is generated and therefore the transistor Qs2 isrendered on. As a result, collector current I₃ flows through the npntype parasitic bipolar transistor Qs2. Then the collector current I₃ ofthe npn type parasitic bipolar transistor Qs2 flows from the drain ofthe MOS field effect transistor M11 to constitute the memory cell 1121.

[0184] In the upper memory cell MC11 shown in FIG. 14, assume that theNMOSFET M11 is rendered off and M12 is rendered on and data is held asshown in the figure. Drain potential of the MOS field effect transistorM11 in the memory cell 1121 is raised to the side of the sourcepotential Vcc by the load resistor R₁₁ of polysilicon having very highresistance value. Consequently, even if the current I₃ drawn from thedrain by the parasitic bipolar transistors Qs1, Qs2 is quite small, thedrain potential-of the MOS field effect transistor M11 is easily loweredand the potential held at high level (H) becomes low level (L) therebythe flip-flop is inverted and the storage information of the memory cellis destroyed. The information destruction is apt to be producedparticularly at portion A in the memory cell array unit 1120 near theperipheral circuit unit 1110, as surrounded by a dotted line in FIG. 15.

[0185] In the semiconductor memory as above described, problems havebeen clarified by the inventors that when the operation source voltageVcc, for example, is high, the bipolar transistor Q₁ is apt to besaturated and therefore the storage information may be partiallydestroyed.

[0186] II. In addition to the above description, the novel informationdestruction mechanism occurs further relating to the input protectiveelement.

[0187]FIG. 17 is a circuit diagram illustrating a portion from input ofthe address signal Ai to the memory cell MC11, for example. The addresssignal Ai is applied to the input pad PAD, and then transmitted throughthe input protective resistor R11 and the MOSFET M6, M7 of the CMOScircuit constituting the input buffer circuit to the word line drivecircuit WD₂, thereby the word line drive circuit WD₂ is rendered on oroff in response to the inputted address signal Ai. M5 designates theinput protective element which protects destruction of the gate oxidefilm of the MOSFET M6, M7 of the CMOS circuit at a next stage from thestatic electricity.

[0188]FIG. 18 shows a sectional view of the input protective element M5and the memory cell MC11 in FIG. 17. The input protective element M5 iscomposed of the n type source areas 120, 120 at the ground potentialformed on the P-well layer layer 113, the n type drain area 121connected electrically to the input pad PAD, and the p type layer 122formed on the P-well layer 113 so as to make the P-well layer 113 theground potential. The memory cell MC11 is similar to that shown in thesectional view of FIG. 16 and therefore the detailed description shallbe omitted.

[0189] Around the input protective element M5 are provided the N typeburied layer 104 formed to surround the MOSFET M5 and connected to thedefinite potential Vcc such as the operation potential, and the N-welllayer 105. The N type buried layer 104 and the N-well layer 105constitute a second minority carrier absorbing guard ring 114.

[0190] In FIG. 17 and FIG. 18, when a signal of negative potential isinputted to the input pad as shown in the figure, other novelinformation destruction will now be described. If the signal of negativepotential is inputted to the input pad PAD, potential of the drain area121 of the input protective element M5 becomes lower than the groundpotential of the p type substrate 100, the p⁺ type buried layer 102, thep type well layer 103, thereby the p type areas 100,102,103 and thedrain area 121 become in the forward bias state. As a result, aparasitic transistor Qs3 is formed using the N-well layer 105 of Vccpotential. the N buried layer 104 formed to surround the inputprotective element M5 as collector, the p type areas 100, 102, 103 asbase, and the drain area 120 of the input protective element M5 asemitter, and base•emitter potential is established, thereby thetransistor Qs3 is rendered-on and collector current I₄ flows. If thevalue of the input signal of the negative potential is small, the N-welllayer 105 being connected to the Vcc potential, the N type buried layer104 can sufficiently absorb the minority carriers (electrons) injectedfrom the n⁺ type layer 120 to the p type areas 100,102,103. If thenegative potential of the input signal is large, however, electrons ofthe minority carriers cannot be sufficiently absorbed by the N-welllayer 105 of the Vcc potential, the N type buried layer 104. As aresult, as shown in FIG. 17, FIG. 18, the parasitic transistor Qs4 isformed using the p type areas 100,102,103 as base, the drain area 121 ofthe input protective element MS as emitter and drain (N⁺ area 106) ofthe MOSFET M11 of the memory cell MC11 holding the shown information ascollector, and base-emitter potential of the transistor Qs4 isestablished, thereby collector current flows from the drain 106 of theMOSFET M11 of the memory cell. As a result, the information held in thememory cell MC11 is destroyed in similar manner to the abovedescription.

[0191] c) Feature of the Second Aspect of the Invention

[0192] A feature of the second aspect of the present invention is thatthe current amplification factor h_(fe) of the parasitic bipolartransistors Qs2, Qs4 using the drain of high potential of the MOSFET ofthe memory cell MC11 as collector is reduced or generation of theparasitic bipolar transistors Qs2, Qs4 is prevented, whereby thedestruction of information in the memory cell is prevented.

[0193] More specifically, the following constitution is possible.

[0194] I. A first impurity introducing layer connected to definite(fixed) potential (Vcc) is formed between a switching circuit (ex. wordline drive circuit), including bipolar transistor and MOSFET, and amemory cell. N type layers 104 a, 105 a shown in FIG. 20, FIG. 21, FIG.22 and N type layer 161 shown in FIG. 23 correspond to the firstimpurity introducing layer, i.e., a first minority carrier absorbingarea 113.

[0195] As above described, the first minority carrier absorbing area isprovided, whereby the current amplification factor h_(fe) of theparasitic bipolar transistors Qs2, Qs4 can be reduced. Consequently,even if a MOSFET with a drain of high potential in the memory cellexists around Bi-CMOS composite switching circuit as a peripheralcircuit, the destruction of information in the memory cell can beprevented.

[0196] II. In order to prevent generation of the parasitic bipolartransistors Qs2, Qs4 more effectively, as shown in FIG. 25, FIG. 26, theP-well layer 103 with the input protective element M5 formed thereon issurrounded by the N-well layer 105 (N type buried layer 104) as a secondminority carrier absorbing area connected to Vcc potential and formed tosurround the input protective element M5 and by the N type buried layer104 b (N type buried layer 104 c) under the P-well layer 103 with theinput protective element M5 formed thereon, and the P-well layer 103with the MOSFET M5 formed thereon is independent electrically of other Ptype wells 103. Otherwise, as shown in FIG. 27, the P type well layer103 with the memory cell MC11 formed thereon is surrounded by the N typeimpurity introducing layer 191, the N buried layer 104 a, the N-well 105a connected to Vcc potential as the first carrier absorbing area 113 andby the N type buried layer 104 d formed under the P type well layer 103with the memroy cell formed thereon and connected electrically to the Ntype buried layer 104 a, the N type well layer 105 a, and the P typewell layer 103 with the memory cell formed thereon is independentelectrically of other P type well layers 103.

[0197] In the above construction, since the moving path of the minoritycarriers can be interrupted, generation of the parasitic bipolartransistor Q4 or both parasitic bipolar transistors Q2 and Q4 can beprevented. As a result, the destruction of information in the memorycell can be prevented.

[0198] Embodiments of this second aspect of the invention will now bedescribed.

[0199] Embodiment 1

[0200] Typical embodiments of the second aspect of the invention will bedescribed referring to the accompanying drawings.

[0201] In the drawings, the same reference numeral designates the sameor like parts.

[0202]FIG. 19, FIG. 20, FIG. 21 and FIG. 22 show an embodiment of asemiconductor memory according to this aspect of the invention.

[0203] The semiconductor memory shown in FIG. 19-FIG. 22 is a static RAMusing a Bi-CMOS type logical circuit in the peripheral circuit, and thebasic constitution is similar to that shown in FIG. 14-FIG. 16.

[0204]FIG. 22 shows the outline of whole constitution of the static RAM1100.

[0205] As shown in FIG. 22, the static RAM 1100 is formed on a singlesemiconductor substrate 1200. Most of the area of the substrate 1200 isoccupied by a memory cell array unit 1120. The memory cell array unit1120 is divided into a plurality of surfaces, and a peripheral circuitunit 1110 is arranged around each divided surface of the memory cellarray unit 1120 and therebetween. Further, a terminal pad PAD 1101 isarranged to the outside. The peripheral circuit unit 1110 comprises aword line decoder and driver unit 1111, a data line decoder andselective switch unit 1112, and a data line pull-up circuit unit 1113.

[0206] In the static RAM 1100 shown in FIG. 19-FIG. 22, in addition tothe constitution shown in FIG. 14-FIG. 16, a first minority carrierabsorbing area 113 obstructing movement of carriers is interposedbetween the memory cell array unit 1120 and the peripheral circuit unit1110. As shown in FIG. 21, the minority carrier absorbing area 113 iscomposed of n⁺ type diffusion layer 191 as a first impurity introducinglayer and n⁺ type buried layer 141, n type well layer 151. The n⁺ typediffusion layer 191 is formed simultaneously with n⁺ type collectorelectrode drawing diffusion layer 109 of the bipolar transistor Q₁. Then⁺ type diffusion layer 191 as a guard band 113 is connected to thesource potential Vcc.

[0207] If the guard band 113 is interposed between the memory cell arrayunit 1120 and the peripheral circuit unit 1110 as above described, itbecomes difficult for the parasitic bipolar transistor Qs2 to be formedbetween the peripheral circuit unit 1110 and the memory cell array unit1120. In place of this, as shown in FIG. 19, FIG. 20, FIG. 21, theparasitic transistor Qs2′ is formed between the peripheral circuit unit1110 and the guard band 113. In this case, before carriers (electrons)straying from the peripheral circuit unit 1110 side to the memory cellarray unit 1120 side attain the memory cell 1121 (MC11), they are caughtby the n⁺ type diffusion layer 191 constituting the guard band 113whereby the effective current amplification factor of the parasiticbipolar transistor Qs2 between the peripheral circuit unit 1110 and thememory cell array unit 1120 can be reduced. As a result, partialdestruction of the storage information caused by use of the Bi-CMOS typelogical circuit in the peripheral circuit can be securely prevented.Thereby the range of the usable source voltage may be large.

[0208] In FIG. 22, the first minority carrier absorbing area 113 isformed only between the word line decoder and driver unit 1111 and thememory array 1120, because the final output stage of the word linedecoder and driver circuit 1111 is constituted by the Bi-CMOS compositeswitching circuit shown by WD₁ in FIG. 19, and the bipolar transistor atthe output stage circuit is arranged near the memory cell 1120. Sincethe final output stage of the data line decoder and selective switchunit (column switch) 1112 is not the Bi-CMOS composite switchingcircuit, the guard band 113 is not installed. However, if necessary, theguard band 113 may be preferably formed in ring shape so as to surroundthe memory arrays 1120, 1120, 1120, 1120 respectively.

[0209]FIG. 23 shows a modification of the embodiment.

[0210] As shown in FIG. 23, the guard band 113 may be the n⁺ typediffusion layer 161 formed simultaneously with the n⁺ type source•draindiffusion layer 106. Also in this case, the n⁺ type diffusion layer 161is connected to the source potential Vcc, whereby similar effect to thatof the above embodiment can be obtained. The guard band 113 in FIG. 14may be formed simultaneously with the collector diffusion layer 109 ofthe bipolar transistor Q₁. In this construction, the process can besimplified.

[0211] Embodiment 2

[0212] A second embodiment of this second aspect of the invention willnow be described referring to FIG. 24.

[0213]FIG. 24 is a device sectional view of the input protective elementM5 disposed near the input pad, the peripheral circuit unit 1110including the Bi-CMOS composite switching circuit, and the memory array1120 including the memory cell. FIG. 24 illustrates prevention ofinformation destruction in the memory cell when a signal of negativepotential is applied to the input pad PAD. FIG. 24 shows structure wherethe input protective element M5 shown in FIG. 18 is added to the left ofFIG. 21.

[0214] A feature of the second embodiment is in that the first minoritycarrier absorbing area 113 and the second minority carrier absorbingarea 114 are formed. As a result, a signal of negative potential isinputted to the input pad PAD, and the potential of the drain n⁺ typelayer 120 of the input protective element M5 becomes lower than thepotential of the p type area 100, 102, 103. Even if the drain 120 andthe p type areas 100, 102, 103 become in the forward bias state, thefirst and second minority carrier absorbing areas 113, 114 exist betweenthe drain n⁺ type layer 120 and the drain n⁺ type layer 106 of theMOSFET 11 of the memory cell MC11 with drain D at high potential (referto FIG. 18). Since the first minority carrier absorbing area 113 exists,the parasitic transistor Qs4 using the drain n⁺ 0 type layer 106 shownin FIG. 18 as collector is not generated. In place of this, theparasitic npn bipolar transistor Qs5 using the n⁺ type buried layer 104a connected to a definite potential such as Vcc potential, the n typewell layer 105 a, the electrode drawing layer 191 as collector, the Ptype areas 100,102,103 as base, and the drain n type area 120 of theinput protective element M5 as emitter is generated, and electrons asthe minority carriers injected from the drain area 120 of the inputprotective element M5 to the p type areas 100,102,103 are caught by theparasitic transistor Qs3 shown in FIG. 18 and the parasitic transistorQs5. As a result, the destruction of information in the memory cell MC11can be prevented. Moreover, since the first minority carrier absorbingarea (guard band) 113 exists between the peripheral circuit unit 1110including the switching circuit as composite circuit of the bipolartransistor Q₁ and the MOSFET M4, and the memory array 1120 including thememory cell MC11 disposed near the peripheral circuit unit 1110,destruction of the memory cell information caused by saturation of thebipolar transistor Q₁ shown in the embodiment 1 can be also prevented.

[0215] Embodiment 3

[0216]FIG. 25 is a device sectional view illustrating a third embodimentof this second aspect of the invention, and shows corresponding parts tothe device sectional view shown in FIG. 24.

[0217] A difference of FIG. 25 from FIG. 24 is in that the n type buriedlayer 104 b as a second buried layer is formed under the p type welllayer 102 with the input protective element M5 formed thereon, andcontacted with the second minority carrier absorbing area 114. Since thesecond minority carrier absorbing area (guard ring) is formed in ringshape so as to surround the input protective element M5, the p type welllayer 103 with the input protective element M5 formed thereon isindependent electrically of the p type substrate 100. Consequently, evenif the drain 120 of the input protective element M5 acquires a negativepotential due to the static electricity, ions injected from the draininto the p type well layer 103 can be securely absorbed in the n⁺ typeburied layer 102 a with Vcc potential and the second minority carrierabsorbing area 114. As a result, Qs4 among the parasitic transistorsQs3, Qs4 caused by the input protective element M5 shown in FIG. 18 isnot at all generated and therefore the destruction of the memory cellinformation caused by the input protective element M5 can be prevented.Since the n⁺ type buried layer 104 a can be formed simultaneously withother n⁺ type buried layers, the impurity diffusion process is notincreased.

[0218]FIG. 26 shows a modification of FIG. 25, and more specificallyshows another forming example of the type buried layer 104 b in FIG. 25.

[0219] As seen from FIG. 26, a feature of the modification is in thatthe n type area 104 c as a second buried layer is newly formed under thep type buried layer 102 so that the p type well 103 with the inputprotective element M5 formed thereon is made independent electrically ofthe p type substrate 100, and the n type area 104 c is formed in a ringshape so as to surround the input protective element M5 and connectelectrically to the second minority carrier absorbing area 114 connectedto the Vcc potential. Consequently, the modification has similar effectto that described in FIG. 25, and further has advantages in that thedevice manufacturing process after forming the n⁺ type buried layer 104c is not different from the Bi-CMOS manufacturing process developed bythe assignee of the present invention.

[0220]FIG. 27 shows another modification of FIG. 25, FIG. 26. A featureof the modification is that the n type area 104 d as first buried layerconnected to the first minority carrier absorbing area 113 is providedunder the p type well 103, with the memory cell 1121 formed thereon, andthe p type buried layer 102. In this case, the first minority carrierabsorbing area 113 may be preferably formed in a ring shape so as tosurround the memory array 1120. Because the potential of the p typeareas 102, 103 with the memory cell 1121 formed thereon, and thepotential of the p type substrate 100 (ground potential), can beseparated using the first minority carrier absorbing area 113 formed ina ring shape and the n type area 104 d, consequently, any of theparasitic transistors Qs2, Qs4 shown in FIG. 16, FIG. 18 is not formedand therefore the destruction of the memory cell information caused bysaturation of the bipolar transistor Q₁ contained in the peripheralcircuit unit 1110 or the negative potential input to the inputprotective element M5 can be prevented.

[0221] Further, this structure has the effect of preventing problems dueto α-ray irradiation. If the α-ray is irradiated, an electron positivehole pair is generated in the substrate. The drain 106 of the MOSFET M11of the memory cell MC11 acquires a high potential, and when theinformation storing state is considered the electrons generated in thesubstrate are attracted by the drain of high potential, whereby thepotential of the drain is reduced and the destruction of information inthe memory cell is generated. In the device structure shown in FIG. 24,however, since the n type region 104 d connected to the Vcc potentialexists between the p type well 102 with the memory cell 1121 formedthereon and the p type substrate 100, the electron of theelectron-positive hole pair formed by the α-ray irradiation is absorbedby the n type area 104 d and therefore the destruction of the memorycell information can be prevented. Accordingly, reliability of thememory to the α-ray is improved.

[0222] As another constitution, adoption of both the n type buried layer104 c in FIG. 26 and the n type buried layer 104 d in FIG. 27 ispossible. An advantage of this case is in that, before forming the ntype buried layer 104, impurity ions such as phosphorus to form the ntype buried layers 104 c, 104 d are ion-implanted into the p typesubstrate 100 through the surface oxide film existing in a part of the ntype buried layer forming portion. And then the nitride film is formedon the p type buried layer 102 forming portion (within the n type buriedlayer 104 c, 104 d forming portion), and using the nitride film as animpurity introducing mask antimony is introduced in the substrate 100and the n type buried layer 104 is formed. And then the p type buriedlayer is formed as above described.

[0223] Although the second aspect of the invention has been specificallydescribed on the basis of the embodiments, the second aspect is notrestricted by the embodiments but various modifications withoutdeparting from the spirit of this aspect of the invention may be made.For example, the guard band 113 may be constituted by a groove or aseparation diffusion layer. Furthermore, the invention may be appliednot only to the Bi-CMOSRAM but also to the MOS memory. For example, inorder to prevent the mutual interference between the CMOS inverter andthe memory cell as shown in FIG. 28(a), the guard band 13′ shown in FIG.15(b) may be provided.

[0224] The invention by the inventors has been described in the case ofapplication to the technique of the Bi-CMOS type static RAM being theapplication field as the background of the invention. However, theinvention is not restricted to this field but can be applied to thetechnique of the dynamic type RAM by the Bi-CMOS technique, for example.

[0225] Effects of this second aspect of the-invention will now bedescribed as follows:

[0226] (1) A first minority carrier absorbing area connected to definitepotential such as Vcc is formed between a memory cell and a switchingcircuit including npn bipolar transistor and NMOSFET, thereby in placeof a parasitic bipolar transistor using a drain with high potential of aMOSFET of a flip-flop type memory cell as collector, p type substrate asbase and source of the NMOSFET of the switching circuit as emitter, anew parasitic bipolar transistor using the first minority carrierabsorbing area as collector and having base and emitter in similarconstitution to the above-mentioned parasitic transistor is formed.Consequently, a positive hole is injected into the p-type substrate bysaturation of the npn bipolar transistor in the switching circuit, andthe newly formed parasitic bipolar transistor with short base width isrendered on, whereby only electrons as the minority carriers areabsorbed by the first minority carrier absorbing area but the parasiticbipolar transistor with long base width using the drain of the MOSFET ofthe memory cell as collector is difficult to be rendered on. As aresult, even if the Bi-CMOS composite switching circuit constituted by acomposite circuit of the bipolar transistor and the MOSFET is disposednear the memory array, the parasitic bipolar transistor using the drainof the MOSFET of the memory cell as collector is prevented from beingrendered on, whereby destruction of the information stored in the memorycan be prevented.

[0227] (2) Since destruction of the information stored in the memorycell can be prevented according to item (1), reliability of the memorycan be improved.

[0228] (3) Since the first impurity introduced to form the firstminority carrier absorbing area can be introduced in the same process asthat of the collector electrode drawing layer of the bipolar transistor,simplification of the process can be attained.

[0229] (4) A second carrier absorbing area formed to surround the inputprotective element and the first carrier absorbing layer is provided,whereby, among a plurality of parasitic bipolar transistors generated byundesirable potential applied to the input of the input protectiveelement, generation of a parasitic bipolar transistor using the n typedrain with high potential of the MOSFET in the memory cell as collector,the p type area as base and the n type drain of the input protectiveelement as emitter can be prevented. In place of this, a new parasiticbipolar transistor using the second carrier absorbing area as collectorand having base and emitter in similar constitution to theabove-mentioned parasitic bipolar transistor is generated. Consequently,generation of the parasitic bipolar transistor using the n type drain ofthe memory cell as an emitter can be prevented, whereby destruction ofthe information stored in the memory cell can be prevented.

[0230] (5) Since destruction of the information stored in the memorycell can be prevented according to item (4), reliability of the memoryregarding the input voltage can be improved.

[0231] The details of the third aspect of the present invention will bedescribed in conjunction with embodiments.

[0232] Embodiment 4

[0233]FIG. 30 shows one of the basic embodiments of the third aspect ofthe present invention, in which the BiCMOS system is applied to adynamic RAM (DRAM) employing a 1-transistor type cell as a memory cellMC.

[0234] In the figure, the sectional structures of nMOS, pMOS and npnBIPtransistors and the memory cell MC are illustrated The information ofthe memory cell MC is read out to a data line DL or rewritten thereinfrom the data line in such a way that a storage capacitance is formedbetween n-type diffusion layers and a plate (PL) and between the n-typediffusion layers and a p-type diffusion layer, and that charges storedin the storage capacitance are controlled by a gate WL to which a wordline signal is applied. Besides, in the memory cell MC shown in thefigure, the p-type diffusion layer is disposed underneath the n-typediffusion layers for storing the charges. This p-type diffusion layerserves to increase the storage capacitance, and also to shield thecapacitor portion from minority carriers attributed to radiant rays suchas alpha particles entering a substrate (to function as a barrier to theminority carriers), thereby to reduce a soft error rate ascribable tothe entry of the radiant rays. Such a structure is detailed as the HiCtype memory cell in “Technical Digest of International Electron DeviceMeeting,” 1977, pp. 287-290, etc. Besides, the phenomenon of soft errorsis detailed in “IEEE Transaction on Electron Device,” Vol. ED-26, No. 1,January, 1979, pp. 2-9, etc.

[0235] As indicated in the figure, the silicon substrate used is ap-type substrate p-Sub. This is intended to employ npn BIP transistorsof high performance and to isolate them efficiently. The impurityconcentration of the substrate is usually selected on the order of10¹⁴-10¹⁶ (cm⁻³) in consideration of the collector-substrate capacitanceof the BIP transistor, etc. Symbols nBL and pBL denote buried layers ofcomparatively high concentrations, which reduce the collector resistanceof the BIP transistor so as to realize high performance andsimultaneously render the resistance values of well regions nWELL andpWELL smaller so as to prevent the occurrence of a latch-up phenomenon.The latch-up phenomenon is stated in “Technical Digest of InternationalElectron Device Meeting,” 1982, pp. 454-477, etc. The impurityconcentrations of the buried layers nBL and pBL are respectivelyselected on the orders 10¹⁸-10²⁰ (cm⁻³) and 10¹⁶-10¹⁸ (cm⁻³). The buriedlayers can be realized by, for example, a method in which they areformed in the substrate p-Sub by diffusion beforehand, followed by theepitaxial growth of silicon and by the formation of the wells pWELL andnWELL, etc., or a method in which they are formed by the implantation ofions at comparatively high energy levels into the substrate p-Sub fromthe front surface thereof, and the details-will be stated later. Eitheror both of these buried layers is/are omitted for some purposes. SymbolCN denotes a heavily-doped layer for lowering the resistance between thecollector node C or a node V_(BB2) and the buried layer nBL. SymbolsnWELL and pWELL denote regions for forming the pMOS and nMOStransistors, respectively. For this example the collector layer of theBIP transistor is made of nWELL and nBL.

[0236] In the construction thus far described, according to this aspectof the present invention, a voltage which is higher or lower than theoperating voltage range of the circuitry is applied as at least one ofvoltages for isolating the individual devices, V_(BB1) (this voltage isgenerally called a “substrate voltage” as it is supplied to thesubstrate through the regions pWELL and pBL) and V_(BB2) (generallycalled a “well voltage”). Whether such a voltage or voltages is/areapplied as either or both of the isolation voltages, may be selectedaccording to purposes. By way of example, under the condition that thecircuitry operates between 0 V and V_(CC) (for example, 5 V), a negativevoltage below 0 V is applied as the substrate voltage V_(BB1) and thevoltage VCC is applied as the well voltage V_(BB2). Thus, the value ofthe substrate voltage V is set so that the substrate and the n-typediffusion layer within the region pWELL, for example, may not beforward-biased even when a minus voltage is applied from inside oroutside the semiconductor device to the n-type diffusion layer by anycause, thereby making it possible to perfectly solve the phenomenon inwhich the minority carriers are injected into the substrate and incurthe malfunction of the circuitry as the problem of the prior art. Thiseffect is especially remarkable in the DRAM of the type storinginformation in the form of charges as shown in FIG. 30, but it is amatter of course that marked effects are obtained also in other devicessuch as a logic LSI, SRAM and ROM. Although, in the above, the wellvoltage V_(BB2) has been set at the supply voltage V_(CC), a similareffect can be obtained even when a voltage higher than the supplyvoltage V_(CC) is applied as the well voltage V_(BB2) according topurposes. Moreover, according to the present invention, the well regionspWELL and nWELL and the diffusion layers lying in contact with them arenot forward-biased, so that the occurrence of the latch-up phenomenoncan be prevented. Furthermore, the reduction of junction capacitancesbecomes possible.

[0237] Although the example employing the p-type substrate has beentaken in the present embodiment, an n-type substrate may well beemployed in, e.g., a case of using a p-n-p BIP transistor. It isneedless to say that, on that occasion, the signs of applied voltagesought to be made opposite. In addition, although the HiC type cell hasbeen illustrated as the memory cell, the embodiment as left intact isalso applicable to cases of employing various planar type or verticaltype memory cells (such as CCC and STC cells) described in “IEEE PROC.,”Vol. 130, pt. I, No. 3, JUNE 1983, pp. 127-135; “1984, 1985International Solid-State Circuit Conference,” Digest of TechnicalPapers; etc. It is as stated before that the DRAM is not restrictive,but that the embodiment as left intact is also applicable to othergeneral LSIs such as SRAMs, ROMs and logic LSIs. Besides, although thevoltage higher or lower than the operating voltage range of thecircuitry is required in the present invention, it can be generatedwithin the semiconductor device by any of methods described in JapaneseUtility Model Registration Application No. 54-82150; or 1976 ISSCCDigest of Technical Papers,” pp. 138-139; etc., and hence, it can alsobe realized without externally feeding a dedicated voltage. Thesubstrate voltage V_(BB1) may well be applied from the rear surface ofthe substrate.

[0238] The embodiment thus far described has referred to the method inwhich the voltage is uniformly applied to the substrate p-Sub or thewell region nWELL within the chip, thereby to solve the problem of theprior art. Next, there will be elucidated a case where unequal isolationvoltages are applied in conformity with the purposes of circuit blockswithin a chip. For example, an input circuit block as to which theinjection of minority carriers is problematic or a circuit block whichneeds to reduce its junction capacitance has the isolation voltagehigher or lower than the operating voltage range of the circuitryapplied thereto as explained in connection with FIG. 30. On the otherhand, in the block of the memory cell in FIG. 30, it is desired that theconcentration of the p-type impurity layer forming the storagecapacitance be heightened to increase the storage capacitance and alsoto augment the shield effect against the minority carriers created bythe entry of alpha particles. It is also desired that devices bemicrominiaturized to attain a higher density and a higher speed. Inthese blocks, the breakdown voltages of the devices become lower.Therefore, the highest or lowest voltage within the operating voltagerange of circuitry is applied as the isolation voltage of such a blocklikewise to the prior art. There will be described embodiments on amethod which applies any desired voltages according to purposes in thismanner, and a semiconductor structure which makes the method possible.

[0239] The techniques to be discussed hereinafter can be applied withoutany change, not only to LSIs of the BiCMOS system, but also to those ofthe conventional pMOS, nMOS or CMOS system. Therefore, various examplesof applications shall be explained without sticking to the BiCMOSsystem.

[0240] Embodiment 5

[0241]FIG. 31 illustrates the application of the above measure to anintegrated circuit of nMOS transistors.

[0242] A construction in the figure is such that a p-type substrate(p-Sub) is formed therein with an n-well layer NW, in which p-welllayers PW₁ and PW₂ are further formed. The nMOS transistors formed inthe two p-wells and the substrate p-Sub are respectively denoted bysymbols nMOS1, nMOS2 and nMOS3. In this structure, voltages V_(BB1),V_(BB2) and V_(BB3) independent of one another can be applied to theisolation layers of the three sorts of nMOS transistors, and they can beselected at suitable voltages according to circuit uses.

[0243] On the other hand, the supply voltage V_(CC) or a voltage whichis at least higher than both the voltages V_(BB2) and V_(BB3) is appliedas V_(BB4) to the n-well layer NW. Although the single nMOS transistorhas been shown in FIG. 3, one well usually has a plurality of nMOStransistors in each of three isolation layers.

[0244] In addition, although two p-wells and the single n-well have beenshown in FIG. 31, this embodiment is also applicable to a combinationwherein a plurality of n-wells are provided and wherein one or morep-wells are designed in each of the n-wells. It is also possible toconstruct all the nMOS transistors in a p-well. Further, the presentinvention can be readily applied to a pMOS integrated circuit merely byaltering the conductivity types of the substrate, wells and MOStransistors and reversing all the potential relations. The voltageV_(BB1) may be applied to the substrate either from the front surface orfrom the rear surface thereof.

[0245] Embodiment 6

[0246]FIG. 32 shows an embodiment in which the third aspect of thepresent invention is applied to an nMOS integrated circuit employing ann-type substrate. Referring to the figure, two p-wells (PW₁,W₂) areformed in the n-type substrate (n-Sub), and nMOS transistors are formedin the respective p-wells. In the illustrated construction, voltagesV_(BB2) and V_(BB3) unequal to each other are applied to the respectivewells PW₁ and PW₂ by applying the present invention. As these voltagesV_(BB2) and V_(BB3), the optimum voltages can be applied depending uponthe circuit portions. By way of example, the ground potential GND can beapplied as the voltage V_(BB3), and a still lower voltage of −3 V as thevoltage V_(BB2). A voltage V_(BB1) to be applied to the substrate n-Submay be the supply voltage V_(CC), or a voltage higher than both thevoltage V_(BB2) and V_(BB3).

[0247] Although only the two p-wells each including the single nMOStransistor therein have been shown in FIG. 32, the embodiment is readilyapplicable to the combination between any desired number of p-wells andany desired number of nMOS transistors. On this occasion, two or morevoltage values as desired may be selected for the applied voltages ofthe plurality of p-wells in conformity with uses. Besides, a pMOSintegrated circuit can be fabricated by inverting the conductivity typesof the substrate, the wells, sources and drains. At this time, positivevoltages unequal to each other are applied as the voltages V_(BB2) andV_(BB3), and the voltage GND or a voltage lower than both the voltagesV_(BB2) and V_(BB3) is applied as the voltage V_(BB1).

[0248] Embodiment 7

[0249]FIG. 33 illustrates an embodiment in which the present inventionis applied to a CMOS (complementary MOS) structure. Referring to thefigure, three n-wells (NW₁, NW₂, NW₃) are formed in a p-type substrate,and p-wells (PW₁, PW₂) are further formed in the respective n-wells NW₁and NW₂. Thereafter, nMOS transistors (nMOS1, nMOS2, nMOS3) arerespectively formed in the p-wells (PW₁, PW₂) and the substrate p-Sub.Besides, pMOS transistors (pMOS1, pMOS2, pMOS3) are respectively formedin the n-wells (NW₁, NW₂, NW₃). In this construction, voltages V_(BB2),V_(BB4) and V_(BB1) are applied to the p-type isolation layers for thenMOS transistors. On the other hand, voltages V_(BB3), V_(BB5) andV_(BB6) are applied to the n-type isolation layers for the pMOStransistors. As three voltages V_(BB2), V_(BB4) and V_(BB1) or as threevoltages V_(BB3), V_(BB5) and V_(BB6), voltages of at least two valuesunequal to each other are applied according to circuits used. By way ofexample, voltages of GND (0 V) and −3 V are applied as the voltagesV_(BB2), V_(BB4) and V_(BB1) while voltages of VCC (+5 V) andV_(CC)+α(+7 V) are applied as the voltages V_(BB3), V_(BB5) and V_(BB6).In this way, the voltages as desired can be applied to the individualisolation layers of the nMOS and pMOS transistors. Although, in FIG. 33,only one MOS transistor has been shown in each of the wells, a pluralityof MOS transistors may well be provided as are necessary.

[0250] In addition, although the numbers of the wells are 3 as then-wells and 2 as the p-wells in FIG. 33, they may be increased ordecreased as are necessary. Further, it is to be understood that thepresent invention is also applicable to a construction wherein thepolarities of the substrate and the wells are inverted. That is to say,p-wells are first formed in an n-type substrate and wherein n-wells aresubsequently formed.

[0251] While the embodiments described above concern the constructionemploying only the MOS transistors, there will now be described examplesin each of which the present invention is applied to an integratedcircuit employing bipolar-transistors or an integrated circuit havingboth bipolar and MOS transistors.

[0252] Embodiment 8

[0253]FIG. 34 illustrates an embodiment in which the present inventionis applied to an integrated circuit employing bipolar transistors.Referring to FIG. 34, three n-p-n bipolar transistors (npn1, npn2, npn3)and one p-n-p bipolar transistor (pnp1) are formed. In a conventionalbipolar integrated circuit, a plurality of n-p-n transistors areconstructed in a p-type substrate p-Sub likewise to the transistor npn3in this figure and are fed with a common substrate voltage as a voltageV_(BB1) from the front surface or rear surface of a chip. When themagnitude of the voltage V_(BB1) is set at the lowest potential GND (0V) in circuitry or at a still lower potential, the plurality of bipolartransistors can be isolated from one another. In the present invention,p-type isolation layers separated from the substrate p-Sub are furtherprovided as indicated by symbols PW₁ and PW₂, and the n-p-n transistors(npn1, npn2) are formed therein. The p-layers have voltages V_(BB2) andV_(BB3) applied thereto. The values of the voltages V_(BB2) and V_(BB3)can be set independently of the voltage V_(BB1). A voltage V_(BB4) isapplied to an n-type layer (nW) for isolating the substrate p-Sub andthe p-layers. When a voltage V_(BB4) (for example, equal to the supplyvoltage V_(CC)) is higher than the three voltages V_(BB1), V_(BB2) andV_(BB3), the transistors npn1, npn2 and npn3 can be perfectly isolatedfrom one another. The p-n-p transistor (pnp1) in the figure can beconstructed by employing some of layers used for forming the transistorsnpn1 and npn2. When the conductivity types of all the layers includingthe substrate are reversed, unequal voltages can be applied to n-typeisolation-layers forming a plurality of p-n-p transistors.

[0254] Embodiment 9

[0255] Next, there will be elucidated an example in which the presentinvention is applied to the so-called BiCMOS structure having both CMOSand bipolar transistors in a chip. FIG. 35 illustrates the embodiment inwhich nMOS transistors (nMOS1, nMOS2, nMOS3) and pMOS transistors(pMOS1, pMOS2) are formed in a p-type substrate p-Sub as in FIG. 33, andin which an n-p-n bipolar transistor (npn1) is further formed. As in theforegoing, voltages V_(BB1), V_(BB2) and V_(BB3) can be independentlyset as the isolation voltages of the nMOS transistors. In addition,voltages V_(BB4) and V_(BB5) can be independently set as the isolationvoltages of the pMOS transistors. The substrate voltage V_(BB1) of thetransistor nMOS3 is applied to the isolation region of the bipolartransistor, but this voltage V_(BB1) can be set as an isolation voltagededicated to the bipolar transistor in the absence of the transistornMOS3. In addition, when a structure similar to that of the transistornpn1 in FIG. 34 is incorporated in the construction of FIG. 35 isolationvoltages unequal to each other can be fed to the bipolar transistors.Besides, a p-n-p transistor can be formed as in FIG. 34. Also, when theconductivity types of the substrate, the wells, the sources and drainsof the MOS transistors, and the collector, emitter and base of thebipolar transistor are all inverted, a p-n-p transistor and a CMOSstructure can be constructed, and the plurality of isolation voltages ofthe present invention can be applied to such a construction.

[0256] Embodiment 10

[0257]FIG. 36 shows an embodiment in which the present invention isapplied to the nMOS portion of a stacked CMOS structure. The figureshows the example which is formed with nMOS transistors on the side of asubstrate, and with pMOS transistors on the substrate by the so-calledSOI structure wherein polycrystalline Si is grown on an insulator film.By combining such a configuration with a p-well (pW) and an n-well (nW),independent voltages V_(BB2) and V_(BB1) can be applied to therespective isolation portions of the transistor nMOS1 formed in thep-well and the transistor nMOS2 formed in the substrate p-Sub. Inaddition, when the conductivity types of the substrate and the wells arerendered opposite, it is possible to form pMOS transistors on thesubstrate side and nMOS transistors on the polycrystalline Si side andto apply separate isolation voltages to the isolation portions of thepMOS transistors.

[0258] Embodiment 11

[0259]FIG. 37 illustrates an embodiment in which nMOS and pMOStransistors are constructed on an insulating substrate written asINSULATOR in the SOI (Silicon on Insulator) or SOS (Silicon on Sapphire)structure, and in which the present invention is applied to such aconstruction. On the insulating substrate, p-type Si (or n-type Si) isformed by crystal growth, whereupon an n-type (or p-type) impurity isintroduced into the p-layer deeply enough to reach the substrate,thereby to isolate a plurality of p-type (or n-type) regions. The nMOStransistors are formed in the isolated p-type regions, and the pMOStransistor is formed in the n-type region, voltages V_(BB1) and V_(BB3)are applied to the respective p-type regions in conformity with the usesof circuits, while a voltage V_(BB2) is applied to the n-type region.The numbers of the p-type and n-type isolation regions in FIG. 37 can beselected as desired, and it is also possible to employ only either pMOStransistors or nMOS transistors.

[0260] Embodiment 12

[0261] Thus far, the various structures for the substrate voltageisolation have been described with reference to FIG. 30 and FIGS. 31-37.Now, there will be described embodiments in which these structures areapplied to semiconductor memories.

[0262]FIG. 38 is a block diagram of any of general memories (including adynamic RAM, a static RAM, a ROM, etc.). Symbol ADR denotes an addressinput, symbol CS a chip select input, symbol WE a write enable input,symbol DI a data input, and symbol DO a data output. The designations ofthese signals are mere examples, and other designations are sometimesused.

[0263] A block I indicates an address buffer as well as a decoder and adriver. A block C indicates a control circuit and a write circuit. Ablock MC indicates a memory cell array. A block SO indicates a sensecircuit and an output circuit. One embodiment of the third aspect of thepresent invention consists in applying the different voltage to thememory cell array MC enclosed with a broken line or to the otherportion.

[0264] Referring to FIG. 39, a chip which includes the two blocksisolated as in FIG. 38 has a built-in substrate bias voltage generator,and the two outputs V_(BBM1) and V_(BBM2) of the bias voltage generatorare applied to the peripheral circuits other than the memory cell array,while a supply voltage VCC and the ground potential GND are applied tothe memory cell array as V_(BBM3) and V_(BBM4) respectively. The circuitarrangement of the substrate bias voltage generator has already beendisclosed in “1976 ISSCC,” pp. 138-139 or Japanese Patent ApplicationLaid-open No. 51-117584. In this construction, by way of example, theisolation region (n-well) of the pMOS transistor of the peripheralcircuit is fed with the voltage V_(BBM1) (+7 V) and the isolation region(p-well) of the nMOS transistor thereof with the voltage V_(BBM2) (−3V), while the n-well of the pMOS transistor of the cell array is fedwith the voltage V_(CC) and the p-well of the nMOS transistor thereofwith 0 V. By supplying the voltage of large absolute values to theisolation regions of the input and output circuits in this manner, thememory becomes stable against the overshoots and undershoots of inputand output signals, junction capacitances (the capacitances between thesources or drains of MOS transistors and a substrate, and thecapacitances between the collectors of bipolar transistors and thesubstrate) can be reduced, and an impurity concentration profile lessprone to soft errors can be selected for the cell array. By the way, thedesignations of isolation voltages for use in the following embodimentsshall correspond to the symbols V_(BBM1), V_(BBM2), V_(BBM3) andV_(BBM4) in FIG. 39 in accordance with the roles thereof.

[0265] Sectional views of embodiments of chips which are obtained forthe embodiment of the chip setup in FIGS. 38 and 39 will be elucidatedbelow. Each of them illustrates the sectional structure of the portionsof the input circuit and dynamic memory cell of a MOS dynamic RAM incorrespondence with the prior-art example in FIG. 29. Although thememory cell is the dynamic cell here, the present invention is similarlyapplicable to an MOS static memory cell and a bipolar static memorycell.

[0266] Embodiment A

[0267] In an embodiment in FIG. 40, the nMOS transistors of an inputprotection circuit (an n-type diffused resistor and an nMOS diode) andan input circuit are formed in a p-well (pW), the pMOS transistor of theinput circuit is formed in an n-well (nW), and a memory cell made of annMOS transistor is formed in a p-type substrate p-Sub. In the presentembodiment, the p-well of the input circuit and the substrate p-Sub areisolated. Therefore, the values of the isolation voltages V_(BBM2) andV_(BBM4) of the respective regions can be independently set. Accordinglyby way of example, the voltage V_(BBM2) can be selected at −3 V in orderto satisfy the specifications of the input circuit, and the voltageV_(BBM4) can be selected at 0 V from the viewpoint of the soft-errorimmunity of the memory cell. A broken line under the memory cellindicates a p-type high impurity concentration layer. In this way, thedisadvantage of the prior-art example elucidated in FIG. 29 can beprevented to provide a stable dynamic memory.

[0268] Embodiment B

[0269] In FIG. 41, only the n-type diffused resistor and nMOS diode ofan input protection circuit are provided in a p-well, and the nMOStransistors of any other peripheral circuits are formed in a p-typesubstrate p-Sub likewise to a memory cell. Besides, a pMOS transistorcan be formed in an n-well. A voltage V_(BBM2) (for example, −3 V) isapplied to the p-well under the n-type diffused resistor and nMOS diodewhich are input protection devices, while a voltage V_(BBM4) (forexample, 0 V) is applied to the substrate p-Sub of the nMOS transistorsof the input circuit and the memory cell. A p-type high concentrationlayer is provided under the memory cell as in FIG. 40. A voltageV_(BBM1) is applied to the n-well. The present embodiment isadvantageous over the embodiment of FIG. 40 in that, since only theinput protection devices are provided in the well, the layout issimplified, and that, since the nMOS transistors other than the inputprotection diode are formed under the same concentration condition overthe cell and the peripheral circuit, the control of threshold voltagesV_(TH) is easy.

[0270] Embodiment C

[0271]FIG. 42 illustrates an embodiment in which a memory cell is formedin a p-well (pW), while the nMOS transistors of an input protectioncircuit and any other peripheral circuit are formed in a p-typesubstrate p-Sub. In the present embodiment, the p-well of comparativelyhigh concentration is provided under the memory cell, and it acts as thesubstitute of the high concentration layer indicated by the broken linein FIG. 40 or 41.

[0272] Embodiment D

[0273]FIG. 43 illustrates an embodiment which employs an n-type layer asa substrate and in which a peripheral circuit and a memory cell areformed in p-wells. Whereas the embodiments of FIGS. 40-42 have thedouble well structure, a well configuration of single layer suffices inthe present embodiment. A voltage V_(BBM2) (for example, −3 V) isapplied to the p-well of the nMOS transistor of the peripheral circuit,while a voltage V_(BBM4) (for example, 0 V) is applied to the p-well ofthe nMOS transistor of the memory cell. In addition, a voltage V_(BBM1)(for example, a supply voltage V_(CC)) is applied to the substraten-Sub. In the p-well to which the voltage V_(BBM2) is applied, only aninput protection circuit may be formed, or peripheral circuits such asan address buffer may well be included.

[0274] Embodiment E

[0275]FIG. 44 illustrates an example in which a pMOS memory cell isformed on a p-type substrate p-Sub. A voltage V_(BBM2) (for example, −3V) is fed to the substrate of the nMOS transistor of a peripheralcircuit, and a voltage V_(BBM1) (for example, +7 V) is applied to then-well of the pMOS transistor of another peripheral circuit, thereby toreduce the source and drain junction capacitances of the pMOStransistor. A voltage V_(BBM3) (for example, a supply voltage V_(CC)) isapplied to the n-well of the memory cell. Thus, it is possible toconstruct a memory in which the input circuit is immune againstundershoots and can attain a high speed, while the memory cell is lessprone to soft errors.

[0276] Embodiment 13

[0277] In the above, Embodiments A-E applied to the MOS memories (SRAM,DRAM) have been described as the practicable configurations ofEmbodiment 12 with reference to FIGS. 40-44. Next, embodiments in eachof which the present invention is applied to a memory by the use of theBiCMOS construction of FIG. 35 having both bipolar and MOS devices willbe described with reference to FIGS. 45-50. Among the embodiments, thoseof FIGS. 45-47 employ epitaxial layers, and those of FIGS. 48-50 don'temploy epitaxial layers.

[0278] Embodiment F

[0279]FIG. 45 shows the nMOS, pMOS and n-p-n bipolar transistors ofperipheral circuits, and a dynamic type nMOS memory cell as viewed fromthe left.

[0280] Under the nMOS memory cell, a p-type buried layer (pBL) of highimpurity concentration is put to intensify the soft-error immunity. Thislayer pBL is also used for the isolation of an n-type buried layer.

[0281] Although the nMOS transistor of the peripheral circuit is formedin a p-well, this p-well can be omitted when a p-type epitaxial layer isemployed. An n-type buried layer nBL of high concentration is providedunder this p-well layer, and an n-layer (CN) of high concentration isadded for feeding a voltage to the layer nBL. Besides, the p-well hasits side surrounded with an n-well thereby to be isolated from a p-typesubstrate p-Sub. A voltage V_(BBM2) (for example, −3 V) is applied tothe nMOS transistor of the peripheral circuit, and the voltage V_(BBM1)(for example, a supply voltage V_(CC)) to the n-well of the pMOStransistor. A common-voltage V_(BBM4) is applied to the isolation layerof the n-p-n bipolar transistor and that of the nMOS transistor of thememory cell. Buried layers nBL and pBL provided under a well serve toreduce the collector resistance of the bipolar transistor, and are alsoeffective to prevent latch-up through the decrease of a substrateresistance.

[0282] Embodiment G

[0283]FIG. 46 illustrates an embodiment in which a memory cell is formedin a p-type substrate p-Sub, and the difference of which from theembodiment of FIG. 45 consists only in a construction under the memorycell. With the construction in FIG. 45, the buried layer pBL of highconcentration might rise and cause the threshold voltage V_(TH) of thenMOS transistor to fluctuate. In contrast, according to the constructionin FIG. 46, a p-type high concentration layer indicated by a broken lineis provided only under a storage capacitor so as to prevent the buriedlayer from rising to the channel part of the nMOS transistor of thememory cell.

[0284] Next, principal steps for realizing the sectional structure ofFIG. 46 are illustrated in FIG. 47. At (a) in FIG. 47, n-type buriedlayers nBL are formed in the front surface of a p-type substrate, and at(b), a p-type buried layer pBL is further formed. Thereafter, layer Epiis formed by epitaxial growth at (c), and n-wells (nWELL) and p-wells(pWELL) are formed in the layer Epi by steps (d) and (e). At (f), layersCN heavily doped with an n-type impurity are formed so as to connectwith the underlying layers nBL. Though not shown in the drawing, theplate of a memory cell, the gates of MOS transistors, the source anddrain layers of the MOS transistors, and if necessary, the emitter layerof a bipolar transistor are thereafter formed. Further, steps such asthe formation of contact holes and metallization are thereafterrequired. In the illustration of FIGS. 45 and 46, the layers CN and nBLreduce the collector resistance of the bipolar transistor. On the otherhand, the contact interfaces between the sources and drains and thewells of the MOS transistors and between the base and the collector ofthe bipolar transistor do not establish the contacts between layers ofvery high concentrations, and they can have their breakdown voltagesheld at degrees necessary for circuit operations.

[0285] Embodiment H

[0286] The above has been an example of the process employing anepitaxial layer, and embodiments which do not employ the epitaxial layerwill now be described with reference to FIGS. 48-50. These embodimentsform a high concentration layer in the place of a p-type substrate at afixed depth by the use of implantation. For this reason, they cancurtail the cost of manufacture as compared with the case of employingthe epitaxial layer.

[0287]FIG. 48 is a sectional view, and the corresponding conceptualdiagram seen from the front surface of a chip is shown in FIG. 49. AnnMOS transistor nMOS1 has its p-type isolation layer (pWELL) enclosedwith an n-layer (CN or n-well), thereby to be isolated from a p-typesubstrate p-Sub.

[0288] The principal steps of a process for realizing the structure ofFIGS. 48 and 49 are shown in FIG. 50. At (a), an n-layer of highconcentration is provided in a substrate p-Sub at a fixed depth from thefront surface thereof by ion implantation. Thereafter, an n-well and ap-well are formed at (b) and (c). The p-well can be omitted in the caseof the p-type substrate p-Sub. At (d), n-layers (CN) of highconcentration are formed so as to reach the n-type buried layer nBL.Steps after (d), for forming MOS and bipolar devices and metallizationare similar to conventional steps.

[0289] As described above in conjunction with many embodiments,according to this third aspect of the present invention, independentvoltages as desired can be applied to the substrate and isolation layerof a MOS device and the isolation layer of a bipolar device, and theoptimum voltages conforming to the purposes of the circuits of thedevices can be selected. Thus, the setting of concentration profiles,the setting of isolation voltages, etc. can be freely done in order tocope with the problems of the undershoots of inputs and outputs,junction capacitances, and soft errors.

[0290] Heretofore, the injection of minority carriers ascribable tosurge noise etc. has arisen at a p-n junction constituting a MOStransistor or a BiP transistor formed in a silicon substrate, and it hasformed the cause of such a problem-as the destruction of a stored signalin an SRAM or a DRAM.

[0291] Such a phenomenon degrades the reliability of the semiconductordevice and is unfavorable.

[0292] The present invention can prevent the phenomenon, is applicableto various products including MOS, BiP, CMOS, Bi-MOS and Bi-CMOS devicesand is capable of enhancements in the reliability etc., so that it canbe effectively utilized.

[0293] While we have shown and described several embodiments inaccordance with the present invention, it is understood that the same isnot limited thereto but is susceptible to numerous changes andmodifications as known to one having ordinary skill in the art and wetherefore do not wish to be limited to the details shown and describedherein, but intend to cover all such modifications as are encompassed bythe scope of the appended claims.

What is claimed is:
 1. A semiconductor device having (1) a memory cellarray including memory cells each having a first N-channel MISFET, and(2) a peripheral circuit including a second N-channel MISFET and abipolar transistor, comprising: a semiconductor substrate having a mainsurface; a first semiconductor region having a first conductivity typein which said first N-channel MISFET is formed; a second semiconductorregion having said first conductivity type and in which said secondN-channel MISFET is formed; a third semiconductor region having a secondconductivity type which is opposite to said first conductivity type; afourth semiconductor region having said first conductivity type formedin said third semiconductor region; a fifth semiconductor region havingsaid second conductivity type formed in said fourth semiconductorregion, and said third, fourth and fifth semiconductor regionsconstitute said bipolar transistor; and a sixth semiconductor regionhaving said second conductivity type formed between said first and saidsecond semiconductor regions, the first semiconductor region beingspaced from the second semiconductor region by said sixth semiconductorregion.
 2. A semiconductor device according to claim 1, wherein a firstfixed potential is supplied to said second semiconductor region and asecond fixed potential which is different from said first fixedpotential, is supplied to said sixth semiconductor region.
 3. Asemiconductor device according to claim 2, wherein said first MISFETcomprises two seventh semiconductor regions having said secondconductivity type formed in said first semiconductor region and a firstconductive strip formed over said main surface of said semiconductorsubstrate via a first insulating film and formed between said twoseventh semiconductor regions, and said second MISFET comprises twoeighth semiconductor regions having said second conductivity type formedin said second semiconductor region and a second conductive strip formedover said main surface of said semiconductor substrate via a secondinsulating film and formed between said two eighth semiconductorregions.
 4. A semiconductor device according to claim 3, wherein saidfirst conductivity type is P-type and said second conductivity type isN-type.
 5. A semiconductor device according to claim 1, wherein saidsixth semiconductor region forms a guard band of the semiconductordevice.
 6. A semiconductor device according to claim 5, wherein theguard band surrounds the memory cell array.
 7. A semiconductor deviceaccording to claim 1, wherein said first N-channel MISFET has source anddrain regions formed in the first semiconductor region, and wherein saidsixth semiconductor region and said source and drain regions of saidfirst N-channel MISFET are regions formed simultaneously.
 8. Asemiconductor device according to claim 1, wherein the bipolartransistor includes a current collector region, and the sixthsemiconductor region and the current collection region are regionsformed simultaneously.
 9. A semiconductor device according to claim 1,further comprising a ninth semiconductor region, of the secondconductivity type, below the sixth semiconductor region.
 10. Asemiconductor device according to claim 1, wherein the sixthsemiconductor region forms a guard band of the semiconductor device, andwherein the guard band is only of the second conductivity type.
 11. Asemiconductor device according to claim 1, said semiconductor devicebeing a static RAM.
 12. A semiconductor device having (1) a memory cellarray including memory cells each having a first N-channel MISFET, and(2) a peripheral circuit including a second N-channel MISFET and abipolar transistor, comprising: a semiconductor substrate having a mainsurface; a first semiconductor region of P-type conductivity formed insaid semiconductor substrate, and said first N-channel MISFET is formedin said first semiconductor region; a second semiconductor region ofP-type conductivity formed in said semiconductor substrate, and saidsecond N-channel MISFET is formed in said second semiconductor region; athird semiconductor region of N-type conductivity formed in saidsemiconductor substrate; a fourth semiconductor region of P-typeconductivity formed in said third semiconductor region; a fifthsemiconductor region of N-type conductivity formed in said fourthsemiconductor region, and said third, fourth and fifth semiconductorregions constitute said bipolar transistor; and a sixth semiconductorregion of N-type conductivity formed between said first and said secondsemiconductor regions, the first semiconductor region being spaced fromthe second semiconductor region by said sixth semiconductor region,wherein a first fixed potential is supplied to said second semiconductorregion, and a second fixed potential which is different from said firstfixed potential is supplied to said sixth semiconductor region.
 13. Asemiconductor device according to claim 12, wherein said first MISFETcomprises two seventh semiconductor regions of N-type conductivity insaid first semiconductor region and a first conductive strip formed oversaid main surface of said semiconductor substrate via a first insulatingfilm and formed between said two seventh semiconductor regions, and saidsecond MISFET comprises two eighth semiconductor regions of N-typeconductivity in said second semiconductor region and a second conductivestrip formed over said main surface of said semiconductor substrate viaa second insulating film and formed between said two eighthsemiconductor regions.
 14. A semiconductor device according to claim 12,further comprising a ninth semiconductor region, of N-type conductivity,below the sixth semiconductor region.
 15. A semiconductor deviceaccording to claim 12, wherein the sixth semiconductor region forms aguard band of the semiconductor device, and wherein the guard band isonly of N-type conductivity.
 16. A semiconductor device according toclaim 12, said semiconductor device being a static RAM.